This is an interesting idea, but could you get it to work?
Yeah, there is already an Adventure Module that has a spellcaster that used Wish to create a commendable Voidstone door that floats within a fraction of an inch of it's frame.
Make a sphere with various hooks inside that if moved or undone shear through a wire that undoes the Quintessence vault, and for good measure set your command word to the lyrics of Bohemian Rhapsody in fourteen languages from inside the sphere (to hide from stuff like hindsight). Assuming the inside has a Forbiddance they have no way of Teleporting in and nothing less than Wishing a new opening will even work in short of the theoretical usages of Contact Other Plane. But assuming they peel back a section the creature nested inside Manifests Forced Dream undoing the last couple of hours, days, or centuries to back when the safe was first sealed giving you time to reassess what you need.
Everything else in between is just extra stuff. Like resetting Spell Traps, intertwining Mystic Fire Searing Corrupt Energy Admixture(fire, fire) Forbiddance to deal up to 32d6 unresistable Fire & Corrupt damage per 10ft moved with high DCed Prismatic Walls in between with multiple Energy Transformation Field to block spellcasting and hit them back, lots of Terrasquaes or Red Dragons slimed up with Quintessence to act as an immortal back up army, one heck of a Diplomacy Check with the local overdeity, and so on.
Edit - Minor correction, it was Dungeon Magazine 153 that had the details of a Voidstone Door.
Getting Past the Voidstone Doors
Because of the nature of the wishes used to place these barriers in the tower, transmutation magic does not affect them (passwall, stoneshape, stone to flesh, and so on). The PCs may easily bypass them with the password (if they learn it), but otherwise spells such as dimension door are a viable way to get past. The down side is that they probably end up in areas 6 or 7. If the PCs examine the voidstone door, they can attempt DC 20 Search or Knowledge (architecture and engineering) checks to notice that it is not fastened to its frame or even touching the tower. Small gaps around the demonic slab could be used to navigate past, provided the PCs have the magical resources or powers needed to alter their sizes or body composition to Diminutive size or smaller, or to become gaseous. Of course, they might also use magic to simply deal enough damage to destroy it. The PCs can also attempt to physically batter it down, though this is by far the riskiest way of dealing with it.
And that tiny gap is enough for Gasous Form to get through so you'll need Permanent Gusts of Wind to counter through arguably Voidstone can touch Voidstone just fine, so if the frame were made out of it you could just seal it.
