Originally Posted by manicmike

Well if I'd done all that it would be time to get serious . It wouldn't be hard to get the head off (with a grinder, if it's already broken) so you can get the old plate off. With the plate removed you'll be able to get at a little of it with pliers or vise grips. If it doesn't move, grind it flat and drill it. Be very careful not to touch the threads with the drill bit or the new screw won't fit very well.
If you do, don't panic: You could tap a new hole and use a slightly bigger screw (but this is a real pain, so be careful instead).