Man, I am so glad I didn't do THAT when I replaced my plugs. I had every other problem - but not that one.
I just saw a set of tools at Sears yesterday that were designed to remove rounded-off bolts. But you have to hammer them on to the head, and I dbout you have the room.
My only other idea is to use a dremel grinding wheel (probably three or four of 'em - they don't last long
) to just take the entire head off the bolt. Then you could remove the coil and have the whole length of the bolt to grab on to with a vice-grip or something similar.