OK, it would need to screw into the derailleur hanger at one point and then clamp at the frame at another point, forming some kind of rigid triangle. At the vortex would an attachment for a pulley that would tension the chain. The pulley would need some pretty hefty bolts holding it to the device.

This could also take the form of a steel plate.

Even then, I'm not sure if there's a way to make the pulley strong enough or even if the bike's chainstays or seatstays have the strength to withstand fixed gear backpedaling.