[ursus]

Been there and did the following:
Start a 1/16 inch reverse drill bit in the center of the slot. Make sure it is properly aligned with the track of the screw and start slowly with the drill in reverse. Pulse the drill speed and flush the drilling with WD-40 every 1/8 inch of depth to a total depth of just over 1 inch. You are now just beyond the end of the screw so shoot in something like PB-Blaster or AeroKroil which will leak down into the threads from above.

Try the screw with a screw driver and, if still tight, repeat the above with a 1/8 inch reverse bit. If that fails, go to a 3/16 inch bit. Usually the twist and vibration of the reverse drill process will work the screw loose and - Voila! - it is out.

You're working from a difficult position so resist the urge to rush this process - time and patience are the watchwords. This is one fitting you don't want to mess up!