Use a drill with a small enough bit to bore a hole into the screw. Then use either a left hand drill bit or an extractor bit to screw it out.
I had to do something similar when a bolt on one of my coil packs snapped right off.
You might get lucky and the first drill bit will catch it and you can just reverse it out. If not then just continue on and then use the other bit for extracting.
Since you don't have a very good view in there I would check like every couple of seconds of drilling to make sure you aren't drilling completely through the bolt.