That’s an interesting idea. There’s all sorts of geared hubs where there’s a small lever on the outside. There’s even an 8 speed fully sealed. Perhaps the question is would it be useful to have all of those options? Also, could one of those hubs handle twice the pressure plus all of the torque?
For touring, I’d go with a bigger wheel. Granted this would force you to be higher up, and weigh a little more, but there’s no replacement for a wheel that goes over bumps better.
A guy named Pietro Biondo from Montreal rode a low giraffe unicycle around the perimeter of North America (more or less) around 1983-4, a distance of over 12,000 miles. It was possibly the longest unicycle ride on record. He used a 26" wheel (with unknown gear ratio), and the cranks essentially just above the wheel for a cycle that was probably a little less than 5’ high. An advantage of this configuration is the ability to use panniers. He had two of those, and a spare tire laid across the top. There was a long article about it in the Unicycling Society of America Newsletter in 1984.
I’m sure Pietro would have dug the option of having a multi-speed, fixie hub.
The challenge with the smaller wheels is that I can find lots of caster wheels that can handle the weight, but not ones where I can weld a gear on without damaging the bearing system. Any tips here on potential wheels would be appreciated. I really want it to be 3 to 5" wheel.
With the gearing system, I suspect the multi gear hubs have issues becoming locked, and there’s also a shifting mechanism just outside of the hub that would interfere with crank arms and pedal action. If you crack these issues, I’ll be near the front of the line. 
I’m not sure about a 5" wheel, but if you want a strong, fat, bouncy 10" wheel, look at moped / goped / go-kart wheels. That way you don’t even have to deal with a freewheel because all of their sprockets are fixed. Who knows? You may find something small enough for your taste.
The hub I posted is a fixie hub; it is already “locked.” also, shifting mechanisms should not be an issue on a giraffe because the cranks are completely seperate from the hub, and out of the way.
Resurrection
Greetings old friends.
I’m be building another of these.
Trials and muni has left me with a compacted spine and blown knees but amazing memories. I ride every year or so to remember a time when my body didn’t ache on it’s own.
Life passes in a flash.
Live on!
Cameron
I have made lots of these, they are great… but do make it so the seat is really low so it can be tiny. They become really hard to ride then and are a real hoot! You can always add a seatpost extender for easy riding. I love mine. The one I have in my loft has a 8" wheel on it.
Roger
