The same thing happened to me the night before Carlisle last year. It was the drivers side front left bolt. I couldn't get the remaining bolt out so I just left it there, drilled another hole directly in front of it, and resecured the seat with a new bolt! The bolt hole in the seat rail is oblong instead of round (at least on my 90 it was), so there should still be room to insert another bolt through and into the new hole...
Good luck!