The Red Menace is a great example of an early geared unicycle with a jackshaft. You could do something like that and replace the top inner cog with a trials freewheel for full time high gear coasting.
Or you could add a geared hub (with freewheel) above the tire like Justin did with his 3 speed build.
Using the nimbus trike hub and bolting on something like the Tomicog would be much cheaper and easier than fabricating your own hub.
I am not sure how you would incorporate a clutch without doing more research as to what clutches are available (my original idea), but if you want it to freewheel a geared hub above the tire could get you multiple speeds, all with freewheeling capability.