I think there's a "good" undead (deathless?). Maybe they could become that, and then shift the campaign setting to staying in the void and fighting all the baddies.
They could meet some rebels. If the party joins up, the rebels will supply them with a weapon crystal that restores 1 CON for each undead they kill. Of course the entire party only gets one crystal so they'll have to share. They ain't cheap, you know!
Somebody on the prime material plane casts planar ally and gets a party member as a random calling. In exchange for the player's help, the character gets to stay on the prime material plane until done. When the mission's done, back to the void he goes. The player will have to negotiate to get the caster (or his friends) to call forth the rest of the party.
The players could trade their equipment for a casting or scroll of plane shift. Or they could steal a scroll from an undead BBEG.