I would nix it. I used flatter pieces of base rock, covered them with sand, and constructed my more shapely rock on top of that. Even if you don't use the flat base rock, rock on glass isn't bad at all IMO
. The potential for problems occur when folks sit rock on top of the sand, and some burrowing creature digs under it and causes the rock to drop on the glass, killing the critter maybe and breaking the glass.
Also, and more critical, I'll bet that rubber is treated with some kinda chemical that would leach into your water and cause even more problems and death.