Idea spawned from motorcyle riding..........
An H-D I used to have had front mounted highway pegs. On a bicycle, instead of one lever actuating both brakes, what if the hand lever actuated only one brake, and the other was actuated by a lever, attached to a short bar, attached by a clamp(or welded) to the down tube? The lever would have a spring to return it to non-braking position(or the brake spring could do that). It wouldn't have to stick out far from the frame, and since when braking you aren't pedaling, no harm in using your foot to operate it. Don't know if something like this exists, but just popped into my head, so thought I'd throw it out there.