Shrooms!

Okay, I finally got them off. Raiyn, your theory on "mushroomed" posts was correct. There are splines around the "seat" of the brake that grip the ends of the bosses when tightened down. These slightly gnurled the edge of the bosses preventing the brakes from being pulled off.

I had to do some serious prying w/ a fat screwdriver using the chainstay for leverage to get them off. The inside of the brake bushings got a bit scratched as they slid over the edge of the bosses, but, whatever.

Thanks Raiyn and others.