toprope question

[astridv]

I have a question for the people with outdoor belaying experience. I have my eye set on this bridge. It's conveniently situated in the middle of nowhere, there's barely even foot traffic, and I figure it might be possible to use the railing for toprope belay. However it's a sandstone railing and I don't know if it's advisable to hang a rope directly around a stone structure or if the friction would damage it. So I'm wondering if there is some kind of gadget that you could fasten around the stone railing, with a metal ring or roll through which the rope would go. If there is, what's it called?