Take a look over on dendroboard if you haven't already:
It's quite a bit smaller then this board, and can move pretty slowly, but there are lot of people over there with a lot of experience, and a pretty good knowledge base, and all kinds of beautiful setups.
I think one of the more popular terrarium/vivarium substrates is known as Atlanta Botanic Gardens Mix (ABG Mix). I can't remember what it is off hand, but a quick google (or just poking around dendroboard) will net you all kinds of info on making and using it.
I've done a couple removable backgrounds for paludariums. it's a bit trickier then just up and building in-tank, but it's a lot more flexible if you ever change your mind, and it's a hell of a lot easier to move.
I think I usually just used a solid styrofoam (white bead kind) background covered in an epoxy/sand mix. I haven't used the polyurethane foam myself (yet), but it's pretty popular, and there are some great looking vivs and backdrops made from it. You wouldn't need anything really strong to attach it to, as once it's cured, the foam will give you all the structural stability you need.
You might even be able to get away with something like putting saran wrap on the inside of your tank (be careful with the corners/joints/overlaps), and just go ahead as if you were building in-tank. Let it cure, and remove the foam, and trim/remove the saran-wrap. I haven't tried this, so there may be some major problems I'm ignorant of.
Sorry to hear you got screwed, but glad that you realize there are other things to be done with the tank. Good luck, and please post pics.