Do you know what kinds of plants you looking into? The main problem is going to be making sure the humidity in the enclosure doesn't get too high. What I'd suggest doing is using small, shallow tupperware containers to root the plants in. I'm assuming most plants you'll be using will do fine in regular old potting soil. You can use the moister soil in the containers while still using aspen shavings in the majority of the setup. That way you don't have to jeopardize your snake's comfort or health but can still try your hand at a planted terrarium. Good luck, and have fun!
I'm not too sure about regular potting soil because of the additives in it... go for an all organic potting soil if you do get that, but you can also use just eco-earth or cocofiber with moss on top...
Try ground coconut fiber. You can get it under the names eco earth or gro brick. It holds humidity well and doesn't have any additives. We used it in all our cages at the Exotic A.R.C.
