3 speed jackshaft
Here's my idea for a 3 speed rig. I think it's original, but maybe I just haven't found it elsewhere yet.
1. Engine output on left side (either frame or rack mount) to behind seat post jackshaft, which moves output to right side and reduces gears as desired.
2. Output from right side of #1 jackshaft connects to 2nd jackshaft lower on the seat post tube. This jackshaft is made from a 3 speed hub. Input goes to the normal hub cog input on the hub's right side, which freewheels. Output is back on the left side, where a sprocket is attached to the hub spoke flanges. Further gear reduction can be gained by a smaller sprocket on #1 jackshaft to larger input sprocket on 3 speed hub. Sturmey Archer hub is available for around $60 new that goes from .75 - 1.33. No disk brake set up though, unfortunately. That would make for easier more secure attachment of left side output sprocket.
3. Output from left side of #2 internal hub jackshaft to rear wheel. Again there is opportunity for more gear reduction. Connect to wheel by disk brake hub, staton or grubee hub, or 9 hole spoke attachement.
That's it. The rest is only details - building jackshafts, attaching jackshafts, adjusting tensions, choosing sprockets and chains, ... oh my!