U can try this - put the RBA in a freezer for like 30 mins, take it out, fire up the coil so that the post heats up then quickly try to unscrew it.
If that fails then I would have taken a drill and broken the screw inside and taken it out in peices but that has a risk of destroying the atty
Take a rubber band. Put it over the hole and then push the key in so the plastic of the band is in the screw with the key. This should make enough friction. Or try a bigger key.