The DM may allow a demilich to access wearable extradimensional spaces…
There's no feat or class support for the supernatural ability phylactery transference of the demilich (Epic Level Handbook 174–7). All the reader has to go on is this description:
Headbands, belts, rings, cloaks, and other wearable items kept in close association with the demilich’s phylactery transfer all their benefits to the demilich no matter how far apart the demilich and the phylactery are located. The standard limits on types of items utilized simultaneously still apply. The sample demilich [absent from the System Resource Document] "wears" bracers of armor +8, headband of intellect +6, pink [and presumably green] Ioun stone, ring of wizardry I, ring of protection +2, and gloves of Dexterity +2. (27)
That said, as mentioned by this fine answer, a DM could rule that when a demilich drapes over his phylactery a belt of hidden pouches (Races of the Wild 173) (5,000 gp; 1 lb.) or a belt of many pockets (Complete Arcane 147–8) (11,000 gp; 1 lb.) the demilich then has access to the belt's contents. If going with this reading then the problem's solved. The phylactery transference ability allows a demilich to receive the benefits of the wearable item as if the demilich were wearing it, and the benefit of a magic belt like the ones listed is being able to stow stuff in it and retrieve stuff from it. Thus the a demilich that's so equipped can pull from such a magic belt the contents of that magic belt, including nonwearable magic items, like scrolls and staffs.
Of course, this also means that anything the demilich can pick up and stuff down its pants is then in the real magic belt. (Magic items affected by the phylactery transference ability aren't actually worn by the demilich; the demilich just benefits from them.) And a demilich may be able to stuff more down its pants than might be expected: A successful dispel magic effect followed by a shrink item effect can make a lot of things—even magical things rendered temporarily nonmagical—little enough to fit into either belt (also see these answers). The demilich must remember to remove the item before the shrink item spell's duration expires, though. (The template demilich grants the creature a +10 bonus to its Intelligence score; it'll remember.)
The belts' demiplanes must be secured, too, lest some muckdweller (Serpent Kingdoms 71) psion manifest the 1st-level psion/wilder power compression [psychometabolism] (Expanded Psionics Handbook 84–5) to achieve Fine size, manifest the 5th-level psion/wilder power psionic plane shift [psychoportation] (124) to enter one or more belt demiplanes, and swipe the demiplanes' contents. Little traps like the 5th-level Sor/Wiz spell symbol of pain [conj] (PH 290) et al. affected by the 5th-level Sor/Wiz spell permanency [univ] (259–60) may secure these demiplanes, but doing so will be expensive due to volume. (The belt of hidden pouches means securing 30 demiplanes, and the belt of many pockets means securing 64!)
Obviously, ruling this way magnifies the demilich's already considerable power. The demilich now has at its (telekinetic?) fingertips immediate access to its entire hoard: its nonwearable magic items in addition to everything from alchemical items to its spellbooks to possibly siege engines—with more storage available by nesting demiplanes. Further, the demilich's now has the ability to stuff itself down its own pants, disappearing down its own pants from whatever plane its on then appearing on a demiplane that has as its access point the magic belt. (Again, size may be an issue, but as the typical once-human demilich is already Diminutive, this isn't a big obstacle.) As long as there's no dimensional anchor-like effects about, the demilich has a near-flawless means of escape from any situation… by taking a move action.
If this sounds totally legit, let me share two storyline ideas:
- Although no demilich would be stupid enough to knowingly put into its magic belt a portable hole (DMG 264) (20,000 gp; 0 lbs.)—the belt of hidden pouches and hidden pockets function like bags of holding (248) (2,500+ gp, 15+ lbs.), after all—just convincing the demilich to perform such a deed may also be a plot unto itself! (Also see this question.)
- A cunning party could mount a Mission: Impossible-style heist that tries to reach the demilich's phylactery by themselves waiting on a demiplane that has as its access point an item the PCs know the demilich will put down its pants. (The PCs, of course, conceal that access point's magical aura with a Nystul's magic aura effect or other countermeasures.) Obviously, as it should, things gets complicated quickly.
Keep in mind that the typical demilich likely would've considered these scenarios. I mean, I did, and I'm pretty sure I don't possess an Intelligence score of 29!
Also, if this doesn't sound legit, read on.
…But a DM could also take a stricter view of phylactery transference
A DM could rule that the above is simply impossible, rendering answers to what the question proposes therefore likewise impossible without, like, a homebrew feat for demiliches only or something.
My understanding is that the mandate for Third Edition supplements was to assume that the reader uses only that supplement and the core rules. Here, the Epic Level Handbook is no exception. It didn't need to worry about wearable demiplane access points. For instance, a glove of storing (DMG 257) (10,000 gp; 0 lbs.) shrinks an item instead, and Heward's handy haversack (259) (2,000 gp; 5 lbs.) doesn't occupy a body slot.
While it's possible the designers still considered the notion of, for example, a demilich adding to its bracers of armor +8 (DMG 250) (64,000 gp; 1 lb.) a bag of holding by using the rules for Adding New Abilities (288), those rules aren't actually implemented anywhere in the core rules or the Epic Level Handbook, and, were they, they'd still need to address this specific case.
With that in mind, to this reader and based on the example items listed in the supernatural ability phylactery transference, it seems frankly clumsy to allow the demilich to use either magic belt in the manner described above because, when it comes down to it, the demilich isn't actually wearing the belt! See, actually using either belt requires real access its physical openings, and the belt is with the phylactery. Draping a belt of hidden pouches on the phylactery is about as effective as putting on it a minor circlet of blasting (252) (6,840 gp; 0 lbs.). In the same way that the demilich can't use the circlet's benefit because the circlet can't hear the command word because the circlet's not really there, the demilich can't stuff things down its pants (or retrieve things) as if the demilich were wearing a belt of hidden pouches because there's no belt of hidden pouches there to stuff things into (or retrieve things from).
(A DM could split the difference and say that an item like a belt of hidden pouches won't work but a minor circlet of blasting would. In that case, the demilich just pays a 50% premium on its staffs and wands and maybe its scrolls so as to add their effects to the worn magic items piled 'round its phylactery; the demilich is unlikely to find this an inconvenience.)
The magic items in the description of the phylactery transference ability provide what are essentially constant passive bonuses. Making a demilich vastly more powerful because it can activate its non-use-activated wearable magic items or otherwise employ its worn magic items that require the magic items themselves be touched seems, to this DM, overkill.
By way of in-game experience, when I last used a demilich in a campaign that I DMed I went with this more conservative reading, and at no time did the players complain that encounters with the creature were too easy.
Note: The Ioun stone (pink rhomboid) grants the wearer a +2 enhancement bonus to Constitution; the Ioun stone (pink and green rhomboid) grants the wearer a +2 enhancement bonus to Charisma. Although the demilich's vast wealth may mean it simply forgot it wore an item that improves its nonexistent Constitution, this latter is actually sort of useful, I guess, to a demilich given its other supernatural abilities. Also, a demilich possesses the at-will spell-like ability alter self, allowing it to assume the form of a tomb mote from Libris Mortis (128) and, while in such a form, wear Tiny pants.