I'd be worried about a cross-bow stabbing something it's not supposed to, like ductwork, light fixture, pipes, your assistant.
They make a small spring-loaded "gun" that they sell at home depot that has 6" glow in the dark plastic darts. The bottom of it has a fishing-reel hooked to it. Seems like a good idea but not sure how well it would work.
I couldn't see an RC car working with 6" fiberglass layed on top of the grid. Even if you got one with tracks I think it would just sink down in it and get caught up in the insulation.
Unless your talking a big one that weighs like 20-pounds...
