The Huni-rex is not a good off-road machine due to the pedals being so low to the ground. You can have pretty bad pedal strike problems even on flat ground. The cheapest solution is a 29er. Maybe with the new triple hole cranks that unicycle.com just came out with. I don’t want to discourage innovation, but there’s not a cheap, reliable solution to the geared hub right now.
A sturmey Archer may work, as they do have some fixed-gear hubs, but durability may be an issue, and keep in mind they only work to gear down, rather than up. If you asked the company, they will undoubtedly tell you it wasn’t designed for a unicycle and they can’t guarantee it will work. For the money that you would use to create a geared setup, you could have 2 wheels, if that’s a solution that’s reasonable for your needs. Geared wheels are extremely cool though. In general I have found custom work to ALWAYS be more expensive than you think in the end, but always rewarding as well. Because the huni-rex is a jack-shaft style, another concern is having the play from 2 different geared systems. I’m sure some other riders who have done more custom work can chime in.
You may be interested in a few wheels that Justin built up:
