I went to the Seattle Bicycle Expo today and saw an interesting type of crank. They’re called Power Cranks <http://www.powercranks.com/>. The cranks spin independently, meaning that the left crank and the right crank deliver power independent from each other. It forces you to pedal in a circle and lift your leg up on the back part of the pedal stroke. Check the web site and the videos on the web site for a better idea. You really need to see the video to get an idea of how these cranks work, and even the video doesn’t give you the whole story.
Now imagine a unicycle with these cranks. You’d have to be clipped in because otherwise it would not be possible bring the cranks up on the backstroke. It would be a very interesting coaster unicycle. Both cranks have standard square tapers and the clutch mechanism is all in the crank so it would be real easy to put these on a standard unicycle.
However, they are really expensive. But imagine trying to ride a unicycle with those cranks.
On a more serious note. There is also an adjustable version of the cranks. See about 1/3 down the page here:
The picture isn’t very clear about the design, but they have a washboard pattern on the crank and the mating piece with the pedal threads. Looks like one of the better adjustable crank designs I’ve seen. Offers very good adjustability and would allow for fairly easy adjustments on the road.