Super Duper Fixie brake. This is {was} awesome lol {Repost sry}

You can lock up your wheel (rear or front) with a caliper quite easily.

A disc brake (with sintered pads) would have an advantage in the wet - but would you want to lock your wheels up in the wet ?

Or a disc brake using the rear hub as the mounting point for the disc. Perhaps some kind of hub with a six bolt or splined mount could be developed by an eccentric entrepreneur. You could even go crazy and put one on the front as well.