I'm just putting this out there for the mental ************ aspect since I don't personally intend to try it:

say you've got rear wheel you really like with a nice freewheel hub. You want to convert it to fixed and don't want to make it a suicide hub. Couldn't you take the whole wheel to a good machinist, and have him lathe down the leading edge of the threads, and reverse thread lock ring threads? It seems like most freewheel hubs have a wide threaded area. There should be enough material there to do this? Is this really crazy, or is it just crazy enough to work?