Spells aren't the same as spell-like abilities
While the first sentence of the supernatural ability shadow spells of the prestige class shadowcraft mage (Races of Stone 120-2) refers broadly to the mage's "figments," the remainder of the ability refers to the mage altering his spells:
The subschool of these spells [that are altered by the ability shadow spells] changes from figment to shadow. A shadowcraft mage can use the altered spell to mimic any sorcerer or wizard conjuration (summoning), conjuration (creation), or evocation spell at least one level lower than the illusion spell. The altered spell functions identically to the shadow conjuration or shadow evocation spell, except that the spell's strength equals 10% per level of the figment spell used. (122)
(Emphasis mine.) Although a case can be made for that starting sentence opening the door to the ability shadow spells working with things other than spells, it's a really hard case to make: otherwise the ability specifies spells—not spell-like abilities, soulmelds, utterances, or other potential figment-production methods—all the way down. With that in mind, it's likely the DM will rule that it's impossible to alter something other than a spell with the ability shadow spells, it just not working on, for instance, a spell-like ability, even even if that spell-like ability is (partially) based on the spell major image.
If the DM can be talked into allowing the effect of lesser invocation flee the scene (Complete Arcane 134) to be the equivalent of major image—the invocation really does say, in part, that "you leave behind a major image of yourself in your place that lasts for 1 round"—, then, of course, the trick does work, but then how exactly it works is up to the DM.
This player wouldn't count on the DM agreeing to such a change, and this DM would be reluctant to agree were a player to make such a case… in the abstract, anyway. (But see below.)
Such a PC would struggle to deal with level-appropriate challenges
The typical route to this trick is difficult and sad. A lesser invocation like flee the scene (Complete Arcane 143) typically requires a creature to have 6 levels of the base class warlock (5-10). A typical wizard must be at least level 7 to be able to cast one 4th-level shadow spell (like the spell shadow conjuration (PH 276)) so as to meet the requirements of the prestige class shadowcraft mage. Shortcutting this with the prestige class eldritch theurge (Complete Mage 57-60) is the way to go, though, allowing a warlock 3/wizard 3/eldritch theurge 4 to enter the prestige class shadowcraft mage. The creature then must advance to level 3 in the prestige class to use the supernatural ability shadow illusion. (Also see Warlocks and Prestige Classes (Complete Arcane 18-19).)
In other words, even if the DM were to allow such a trick to work, level 13 (or even level 16 if the prestige class eldritch theurge is unavailable) is usually a long ways off, and until then (and, perhaps, afterward, too), the character may be a bit of a burden. (In a campaign wherein the PC isn't a burden and with an agreeable DM it still might be better to take shadowcraft mage after finishing eldritch theurge, theurge builds being notoriously difficult to fill out and theurge builds benefiting more from advancing both classes typically necessary for entry, even if a cool DM-approved trick available.)
However, were a player to come to this DM and demand to play such a character (and couldn't be talked out of it), this DM would probably end up allowing it—it's the player's game, too, after all.