Woodman components make some too, a disc model and a non-disc. I can't find pricing on it though. The non fixed hub runs for only $54, but I have no idea what the fixed hubs would be worth. Considerably more, I would guess.

I think there would be some problems with interface. If the hub isn't solidly meshed, you would feel some play when you switch between forward and reverse pedaling. Maybe use a "Kickass Cog" from Endless Cycles - they have a super wide base that would probably help.

I've always wanted to try this setup, but am pretty happy with my normal hubs for the time being.

Another option would always be the level hubs. Nice quality, and eeasy cog changes - probably simpler than a cassette, given the bolt design.