I had the bottoms of my mirrors split, but if the nut comes all the way off the bottom of the bolt, it's very tough to get it restarted through the little slit. I wound up cutting the rubber off the whole back of the mirror to get the bolt replaced.
I also switched the bolt for a metric hex bolt, flipped from stock, so the head is on the bottom and the nut is on top. This lets me use a 90 degree hex on the bottom, clearing the faring, and a socket on top without clearing issues. I'm happy with it this way, but it's uglier.