I for one am always happy to revisit these as the PRP threads are some of my favorite things on this site.
When I first read your "magical-post-apocalyptic" suggestion, I immediately though it might make a better fit for the "magic combat" layer of Acheron; but I am curious to see where you might go with this idea.
-Do you see mutated monsters roaming the sides of the Gehenna volcanoes?
-Do you see the area(s) of magical radiation spreading over the plane?
-Are you envisioning these "stolen" pieces appearing as floating earthbergs suddenly appearing in the Gehennean sky? (And perhaps striking down like meteorites?)
How do define the themes of Gehenna and how do you see this concept tying in? (You mentioned the ecological devastation; does it work thematically on any other level?)
Resurrecting this conversation...
If you combine the "stealing pieces of other worlds" and the "ecological devastation" ideas, you end up with a nifty post-apocalyptic feel to the plane. Along those lines, there could be areas of the plane that are steeped in magical "radiation," perhaps the remnants of ancient magical wars or powerful experimental spells gone wrong.