I wonder about this too. From my initial terrarium style set up , the background is a slab of compressed styrofoam that has been carved,(dissolved?) for contour. Question is what next? It has some painted surface material - very well bonded/melded to its surface. So what about toxicity? there are baby -proof paints... That other picture you posted of the super corner-unit cage really does fire up the imagination for objects to be embedded in the background: patterns, perches, food, water falls, planters..
I've just moved my Veiled up to a large all screen cage with lots of plants, but still it lacks something.
I hope you get some more responses, Wendy