I think it would be ok (but that's just me). Snakes climb over rocks all the time in the wild, so I can't see why ours can't. I would just make sure there aren't any really sharp points, and if there were - I would file them down. I would be very vigilant about checking to make sure that your snake didn't get cut. If you feel secure with the decision then go ahead.

Oh and you may need to monitor to make sure that rock isn't absorbing too much heat! You don't want to accidently make a heated rock! It may be just too much of a hassle, so it up to you!