use a dremel or trangular file or small saw or something similar.
cut a notch in the bolt, a nice deep one so you can reach in with a nice fat flat-head and have plenty of grabbing room for lots of torque. If necessary, take a driver bit and put it into the appropriately sized torque wrench for increased power. If that don't get it out, you've got two options: If it's in place with a nut on the other side, just grind off the head and knock it out. If there is no nut on the other side (which I highly doubt), center punch and drill right into the bolt. Keep drilling with larget bits (like removing a rivet from an AK), but once you see rust inside, you know you're in the threading [and if no rust, then the thing must be superglued or melted(?!) in place]. now take a tap and thread right over the remenants of the old screw. Easiest way, if there's a nut on the other side, is just to dremel or grind the head off.