I've seen the back glass idea, and it looks pretty cool, if you like it. Also, no guarantee they'll stay on the glass. I've had them spread pretty quickly under PCs, and now that the lighting is being upgraded I'm reluctant to put them in. You might try a small rock of GSP in a darker area, and be sure to have smaller rock or rubble pieces around for it to spread onto. What I did was to put rubble in the direction of growth [generally toward the light] and cut the purple mantle with an exacto knife when it spread onto the new rock. I'd trade it for whatever I could get at the LFS - small hardware, food, etc. Once it's established over a substantial area, however....
As others have posted here, good luck!!