Ben has tried to get a mech brake working on an Oracle hub setup and was unable to find one that wasn’t too thick. See the first 3-4 pages of this thread: Disk Brake choice for new Nimbus hub
I tried putting a Tektro Aquila on a MountainUni crank based setup but it was about 2mm too wide (cleared the crank but not the spokes). I’d be very happy to learn of a significantly thinner mechanical caliper.