I think I would soak it in oil for a while, dry it off, heat it up a bit to help loosen the rust then grab the shaft of the screw with a pair of vise grips and wiggle the screw back and forth until it breaks free then unscrew it. If the vise grips bung up the shaft too much to get it out of the frame, cut the shaft and remove from inside the frame. Nothing like a BFFI (brute force and flipping ignorance) approach to a problem, eh what?
