I've just stuck a Blackburn MTN-1 on the back of my RoadRat, which I know isn't the best as it was designed for 26" wheels, but it fits, so all's good. I originally bought another rack before I through about it properly, and that one hasn't a chance of fitting, and I'd rather not try and bend it as it uses tubes instead of solid rod as on the Blackburn.
Anyway, it still took a fair amount of bending on the Blackburn to get it to clear the disc caliper. If you've got the Tektro discs that Cotic sell with the complete bikes, then it's also an idea to try and use a smaller button head screw for clamping the brake cable into the caliper… this actually makes a lot of difference to how much bending you need to do!
Another option would be to space out the rack on the side with the disc brake, so that it clears the caliper.