Another idea is to get one of those special security screw tools. They are designed to tighten\loosen those flat topped fasteners with the 2 small holes on the head. Then drill your own little holes.

You could also get a small grinding wheel on a dremel and cut a slot in the top. Then use your screwdriver.

Drilling into the top with a left handed drill bit may do the trick too.

I've also chiseled of the head and used easy outs to get the shaft out. Once the head is off, there is no tension holding the bolt in, and it comes out easily.

I've apparently worked on a lot of crusty stripped stuff in my day..