In the Past a Shimano attempt, Positron .. front freewheel crankset BB combo.
You don't freewheel on a Unicycle .. it is the original fixed gear.
Schlumph makes something like this, I think. I think it's only 2 speeds.
there is a planetary gear ratio change in those unicycle hubs
direct and overdrive, but no freewheel.
site says a 24" wheel, in the high range, acts like a 36" wheel, for cruising speeds.