Instead of trying to figure out how to make glass (of all things) indestructible, how about just make a removable iron cage similar to the one that is guarding the entrance to the hill. No hammer will crack it, and someone would have to spend some time and much, much more effort to get at Howard's Rock.
It simply needs to be about a 6 X 6 cage that has removable four sides that anchor into the concrete. Have the anchor points recessed so that it doesn't hinder foot traffic on game day.