Yes, but it's more complicated than the question makes it out to be
Strictly speaking, a creature can take the feat Assume Supernatural Ability (launch spawn (Monster Manual V 115)) (Savage Species 30–1) then, if manifester level 10 or higher, use the 4th-level egoist power metamorphosis [psychometabolism] (Expanded Psionics Handbook 116–17) at al. to take the form of a madcrafter of Thoon (MM5 114–16) and spit countless scythers of Thoon (116–18), but, practically, doing so may not be as elegant as the question supposes.
The supernatural ability launch spawn begins by saying
A madcrafter of Thoon usually takes a full day to give birth to a stormcloud of Thoon or a scyther of Thoon. When threatened, however, it can create constructs far more rapidly, then expel them in globules of caustic spittle. Once per round, as a swift action, a madcrafter of Thoon can spit a stormcloud of Thoon or a scyther of Thoon into any unoccupied square within 60 feet. (115 and emphases mine)
Using the launch spawn ability to give birth to a scyther of Thoon usually takes a madcrafter of Thoon 1 day. Methods beyond this answer's scope exist to extend the metamorphosis power's duration beyond its normal 1 min./level so that it's possible for a PC to use the launch spawn ability in this usual way, but that's not the question's concern.
Instead, to take a swift action to use the launch spawn ability, the DM must rule on what threatened means in the launch spawn ability. You, clever reader, may think of more, but, broadly, I imagine two rulings:
The DM rules that here threatened means that the creature is tactically threatened. The creature is threatened when it's in a position where another creature can make against it an attack of opportunity. This ruling favors the plan in question, but it favors even more actual madcrafters of Thoon. If, in a nonstressful situation, a nearby ally can trigger a madcrafter's quick expulsion of scythers by wielding a weapon, madcrafters will have long ago realized this and exploited it. A madcrafter must still find a way to heal itself so that it needn't spend its precious quintessence making scythers (see below), but afterward it can set up a staging area from which to spit.
While still largely able to take actions normally (if the launch spawn ability takes only a swift action to use), a madcrafter can launch 1 scyther per round hence 10 per minute hence 600 per hour hence 4,800 per 8-hour workday—that come with 9,600 masterwork scythes with a resale value of over 1.5 million gp. To be clear: That's per madcrafter. The issue then becomes that the madcrafters would have been doing this for nearly their entire existence. (Seriously, they're Int 19. If it's the first thing we'd do, then it's the first thing they'd do.)
Designing a campaign setting with this in mind is interesting. The Allies stormed Normandy with 130,000 troops; 9 madcrafters can make an army that size in about 3 days. A lone madcrafter endangers a small continent. The Endless Blade Armies of Thoon become an apocalypse to be averted or survived.
However, I suspect that most campaign settings are not designed with the idea that madcrafters of Thoon employ their launch spawn ability with such rigor and vigor. So, in most campaign settings, after ruling that threatened means not emotionally, morally, or spiritually threatened but mechanically threatened, the DM must grapple with that implication, perhaps leading to retcons and strained verisimilitude. (N.b. I've not met a DM who likes retconning, nor have I met a DM who wants a setting to lack verisimilitude.)
In short, a DM who rules this way empowers a PC like the one the question describes—and others who follow the same path and others who have already followed the same path—to such a degree that the campaign setting must address it. In any large city of 5,001–12,000, at least 9 inhabitants can potentially tread this path fairly easily (DMG 139). And, from an optimization perspective—and that seems to be the question's perspective—, if the DM doesn't mechanically or narratively gate this path and those 9 folks are not treading it, then they will or should be doing something as or more powerful.
The DM rules that here threatened means that the creature's being menaced or pressured. The creature is threatened when it's in danger, challenged, stressed, or whatever state of mind the DM determines that the creature must be in to feel actually threatened—manufactured stressors aren't enough. This could still be enabled by, for instance, a Thoon elder brain (MM5 121–4) that deliberately subjects its madcrafter minions to a constant barrage of terrible trials. However, I can imagine that inuring madcrafters to all but the most heinous heretofore unknown tortures, making it so madcrafters still mainly use the launch spawn ability in the usual fashion, quick use relegated to abnormal circumstances—like when powerful PCs attack a madcrafter's lair.
This ruling makes it so that the DM tells the player when his PC who assumes the form of a madcrafter of Thoon feels threatened enough to use quickly the launch spawn ability. I know of no player who likes the DM to dictate the player's character's state of mind, so, ideally, the PC should for some reason always feels threatened. I imagine role-playing such a PC would be taxing, especially if the catalyst for the PC's personality is trauma or mental illness. Role-playing a PC with, for instance, acute paranoia specifically so that, at level 10 and beyond, my PC can take a swift action to use the launch spawn ability doesn't sound like a pleasurable way to spend my leisure time—especially if the campaign begins at level 1—, but it may to someone else.
Anyway, after some further description, the the launch spawn ability concludes, saying
A typical madcrafter of Thoon has enough stored quintessence to safely use its launch spawn ability twice per day. If it uses launch spawn a third time, it loses its fast healing ability for the rest of the day. The fourth [i.e. the second with no stored quintessence] and subsequent times it uses launch spawn, it takes 20 points of damage. (ibid. and emphases mine)
A creature that uses the metamorphosis power or other ability to take the form or a madcrafter of Thoon will not be, in most campaigns, a typical madcrafter of Thoon therefore shouldn't have any stored quintessence. Thus, while in madcrafter of Thoon form, either the DM makes available via plot some quintessence to store then use (see How Quintessence Works (107), noting that this quintessence is different from that produced by the power—yes, a thesaurus is in order) or after the first use of the launch spawn ability the creature loses its (maybe nonexistent) fast healing for the day and each additional use of the launch spawn ability deals it 20 points of damage.
To be fair, in a campaign that normalizes the Assume Supernatural Ability (launch spawn) feat as fair and fully functional, being dealt 20 points of damage is likely a pittance for literally anything, much less a 9 HD razorbot with laser vision. So merely taking damage from the launch spawn ability is probably less of a concern than having to play a PC whose powers get weaker with medication or therapy, but a way to mitigate that damage needs to happen nonetheless. (Doing so is largely beyond this answer's scope, but get started with the 4th-level Clr spell delay death [necro] (Spell Compendium 63).)
Issuing orders to scythers
The section Ecology of the scythers of Thoon does, indeed, say, "After being given enough quintessence and metal ore, madcrafters of Thoon spit out scythers that then await orders from any telepathic creature of Thoon" (118). Further, a scyther's lone language entry is "understands telepathic commands" (117).
Normally, a madcrafter of Thoon issues orders to new scythers with its supernatural ability telepathy 100 ft. (MM5 114, q.v. MM 316). The psicrystal Special telepathic speech grants a psicrystal that's owned by a level 5 or higher manifester an extraordinary ability that allows "the psicrystal [to] communicate telepathically with any creature that has a language and is within 30 feet of the psicrystal, while the psicrystal is also within 1 mile of the owner" (XPH 22). Thus an appropriate psicrystal that assumes the form of a creature of Thoon should be able to issue orders to a scyther.1
Once a new scyther has been issued telepathic orders by a creature of Thoon, that scyther obeys those orders until issued additional orders the same way. A scyther literally can't receive orders any other way except telepathically, though, as it lacks any other "language" to receive orders in. A scyther ordered to obey another creature that lacks telepathy presumably follows the last order it was given (which may see it do nothing at all).
This DM would rule that a scyther will follow that another creature's telepathic orders if the order to follow another creature was issued telepathically by a creature of Thoon. (Ordering your scythers to obey only you or another trusted creature is a smart move anyway. Otherwise, any creature of Thoon can totally telepathically swipe your scythers.)
On the other hand, another DM may rule that telepathic orders can be issued to a scyther only by a creature of Thoon specifically. Such a ruling makes army management more complicated as then the Endless Blade Armies of Thoon can't be commanded by telepathic subordinates not of Thoon. Another DM may rule, instead or in addition, that a scyther always follows the telepathic orders issued by all and any creature of Thoon, even if the scyther was told not to follow another creature's orders. (I imagine that DM saying, "Honestly, what did you think that they 'await orders from any telepathic creature of Thoon' meant?") That sounds kind of shady to this player and this DM, but if I can imagine it, someone else can, too, so it's better to ask first.2 You really don't want your carefully expectorated army of razorbots with laser vision turned on you the first time you face in battle madcrafter of Thoon, real or ersatz.
1 Without a psicrystal, the the telepathy ability is typically gained from either the prestige class mindbender level 1 class feature telepathy (Complete Arcane 55) that grants the supernatural ability telepathy 100 ft., or the telepath level 5 alternative class feature telepathic communication that grants the supernatural ability telepathy with a range of 5 ft. per manifester level. (N.b. the metamorphosis power is an egoist power.) Like many things in D&D 3.5, though, there are a host of others, like just taking again the feat Assume Supernatural Ability, for instance.
2 I can imagine a DM using this ruling to explain why madcrafters of Thoon work together rather than, for example, one madcrafter establishing a personal empire by encoding all the scythers to its individual will. (If scythers always obey the last telepathic order issued by those of Thoon, then a telepathic creature of Thoon betraying other telepathic creatures of Thoon becomes far riskier.) I wouldn't like that ruling, but I'd understand it.
Note: This answer omits discussion of the stormclouds of Thoon (118–19). While a stormcloud can also be brought forth using the launch spawn ability, a stormcloud's sentience (such as it is) makes it less useful as a minion than a scyther. For another wacky use of an ability possessed by a creature of Thoon, see here.
