Someone should start making a clamp on, two piece hub adapter like the sprocket adapters but made to take a disc brake rotor. That way you could run a sprocket on the left, like Paul is doing, and a disc brake on the right.
The only problem I can see is that with the 6 bolt, ISO pattern most rotors use you might have spoke interference. I might experiment with this some day.
Some guys have adapted a disc brake rotor to mount with the sprocket but in every case I've seen they have to use a small, 36T sprocket to have enough clearance for the caliper to miss the chain. Or mount the caliper where it doesn't make full pad contact with the rotor.
A clamp on adapter would make putting disc brakes on either the front or rear wheel easy. You'd still need to fabricate a caliper bracket but that wouldn't be hard.
Tom