I tried to get hold of R&M but don't think I managed to get past the marketing people who didn't have a.... suggested that no-one in their right mind would use disc brakes.
Anyway, google found "long john" who had a birdy with a rohloff disc brake and front drum brake in what looks like a custom job. Apart from that single example no-one else seems to have done anything similar.
So.......anyone here done it or know if there are after market parts/frame builders/machinists that can solve the problem? Front AND rear disc brakes, that is.