I may have found the answer: the aptly-named sex bolt (
http://www.boltdepot.com/Sex-bolts-and- ... crews.aspx). Tres sexy!
I'm going to stop by Lowe's and grab some tonight. I'll post whether or not it works, along with pictures of the whole process, soon!
UPDATE:
Ok, the sex bolt that fits the bolt on the bike (M8x1.25) is too large to fit through the eyebolt.
I'm still adamantly opposed to replacing the stock bolt in the bike except as a last resort, though, so my next attempt will be to find an L bracket that I can bolt to the coupling nuts, then to the fender. That'll be a tonight thing.