I got this idea, and remembered reading something vaguely similar. Nothing on search though.

Flip-flop hub, singlespeed freewheel on each side.
Chainring on both crank arms. Smaller chainring on left side.
Pedal forward and you go forward, left freewheel coasts.
Pedal backward and you slow down, right freewheel coasts.
Don't pedal and both freewheels coast (normal coasting).

Anyone tried this? Any info on it? Would it even work?