Re: Chain Tensioner Pedal Side
you might need to put a "stop" tab on it or something so it doesn't rotate. something as simple as a bolt would work, so it catches under the frame.
on a freewheel there'd be no problems with your design, but a coaster brake could give you problems. a lot of torque is applied to the chain when pedaling and hard braking which could cause it to slip. and if it slipped, you'd not only have a loose chain, but it'd loosen the axle nut as well.
other than that, it's a great idea!