The Nimbus Trike hub is decent quality, not sure how many “pawls” it is, but I doubt we’ll put enough pressure on the hub in forward rotation to overtax the mechanism.
In terms of design, first think of this as a bike freehub body, there are sealed bearings press fit into the hub body which “carries” the hub on the spindle, then there is a second set of sealed bearings (40mm OD) which carry the frame on the spindle. Between the bearings is a ~6mm spacer. A “C” clip keeps the outer bearings on the spindle, retained in a groove machined into the spindle.
The spindle is square taper, smaller diameter than an ISIS unicycle hub spindle, so we pretty much need to use the hub and bearings as provided. I don’t have the bearing shims, but they should work fine. I have two crank lengths, 150 and 125. I’m not sure which set I’ll start with, suggestions??
Per Roger at UDC UK, the rotor spacing is slightly different from the Oracle, so I may need to space the IS caliper mount with washers. It was a PITA to dish the 24" wheel, I ended up with very high spoke tension differential between sides, it works, but I think it’s easier to dish a larger diameter wheel.
I’m running a 185mm rotor because it’s what I had available. I’ll be using my extra MT 2 disc brake, KH underseat brake hanger, and a standard KH grab handle. I may add one of my handles if I don’t have a ton of UPD’s; I don’t want to trash a handle with it hitting the ground a whole bunch.
I have dreams of this thing working out, but I know I need to temper my dreams with reality, so I will be wearing armor, planning to start on a slight incline, and a soft surface like grass. Should be fun 
I am camera shy, so I won’t post video unless I’m a superstar (unlikely), but I will post pics and feedback.
So about that 120mm spaced hub you mentioned…is it a disc hub?