I sort of know a guy who has done that with a Sachs hub. He mailed the thing to Paraguay, probably to be modified by that same guy. He put it on a steamroller, and I test rode it. It only has two speeds, and the low gear has a lot of slop from the hub. I didn't try skidding on it, because I didn't ask. It felt solid though.
Someone really needs to make a purpose-built multi-speed fixed hub. It would really be great for urban commuting. I bet that it would be possible to get all or most of the slop out of the hub if it was made to be fixed, rather than converted.