Interesting. I like the "cave" idea. Your definitely could make it work very well. There is another free site that is good for these questions (sorry APC).
www.DendroBoard.com On that site, there is a very, very cool DIY rock-wall construction by StevenBonheim that involves styrofoam and tile grout. It is very realistic. Check it out, if has answered a ton of my own questions.