I have been thinking along the same lines about the jackshaft for my stretch. Stanton Inc. sells a Giant Stiletto jackshaft with a left hand freewheel for an engine. 20T freewheel on left, 20T fixed for wheel drive on right, and 20T freewheel outside right for pedal chain. Its all on a 5/8" shaft that is 6 3/4" long. Goes for $74. Then a nice mounting plate with two u-bolts could be fitted with either a solid pipe with a bearing on each end, or I was actually thinking of doing two small rings on each side of the mounting plate that hold two bearings each, so a total of four bearings. You would also need some sort of L-bracket that uses a u-bolt to attach to the seat post and keep the whole thing centered. Good luck and as soon as I get to work on mine, I'll post some picts. Believe me, these stretches benefit from multiple gears. Husky has a Nexus internal 3spd with 12g spokes and solid steel rim for $109. L8R.
Whoops, their Black alloy rim with 3spd is $109, the steel is only $99.