I would drill into the screw head until it comes off completely, then the  
key slip should just lift out as though the screw were a guide pin. 
You would have to look at the other screws to make sure the threading
is only in the key slip part.  Another way is a stripped screw remover
kit you can get at the hardware store.

--Respectfully, 
Chris Rawson,CPT,RPT


