Originally Posted by kb1jki
...(Geared jackshaft design)...
I still reckon a multi-speed gearbox that is small, light, drives both ways in all gears, doesn't have too much slack/backlash and is shiftable without losing drive would be pretty hard to come up with. Most bike geared hubs can only drive in one direction because they "cheat" by using roller clutches to avoid having to mesh and unmesh gears.
Single-speed jackshaft unicycles have of course been done, as I'm sure you know (Pete Perron's are the highest-profile ones from a few years ago).
Given the gearbox would have to be bespoke, I'd suspect it would be just as costly as a Schlumpf. Somebody suggested using a NuVinci CVT gearbox on a jackshaft, but I think they asked NuVinci about driving in reverse and they suggested it wasn't really meant to do that. And they weigh a silly amount! Although a CVT unicycle would be the holy grail of gearing systems - no nasty lurches on shifting.