I've got my Tomicog bolted onto a Surly front disk hub, which has been respaced to work as a rear wheel. Sorta like this...
http://www.fixedgeargallery.com/arti...mson/surlyhub/

With the wheel dished symmetrically, I get about a 46 mm chainline. That works for me since my crank chainline is 45.5 mm. A true rear disk hub will give you a wider chainline than that. But there is really no reason you can't get the proper chainline, given the multple ways you can adjust it. (1) you can redish the wheel and adjust axle spacers, (2) move your bolt-on disk cog towards the dropout with some washers, (3) move the chainring with chainring spacers, (4) or change the BB spindle. You can usually cure a severely f'ed up chainline by adjusting the first three things without having to buy a new bottom bracket.