I like the idea except for this bit:
If the player cannot escape in time then permadeath happens.
No matter which way you spin it, this is not a fun mechanic, and I don't feel conveys a sense of tension that is conducive to wider gameplay.
In my opinion, such a "temporary pocket loot-bag system" would function by having a VERY short lifespan, and a bunch of goodies for the player to explore within... Tuned in such a way that the player cannot possibly explore everything before the hole collapses, at which point the player would merely be ejected back out into hyperspace, taking a hefty CR penalty perhaps if they didn't exit the hole manually.
The whole idea sounds interesting, but also like it's undecided between being a fast-paced, very tense mini-boss/side quest; a weird sort of timed puzzle game; and a chilled-out "treasure room" kind of deal. Whereabouts on the dial do you envision the needle to be pointing?
Also why are there Domain goodies and Remnants in this freak pocket system? Domain-era leftovers don't exactly move around, and Remnants are kinda secluded to the systems they inhabit (outside of Nexerelin) so if these white holes can just appear and disappear at random it doesn't really make sense for such content to be contained.