After a bunch of iterations during three months of furious design/build/test, I've finally arrived at what feels like a good design for a multi-purpose joint that can work for (at least) everything from the wrist back. This version has a number of virtues:

1. .750" through-joint hole: there's a good-size hole through the hub for running wires or other plumbing. This size was a good compromise between size and weight of the assembly, through-hole capacity, and rigidity.
2. Large reduction gearing: Torque is our friend, so this design allows for the use of reduction from 2:1 to 6:1 with a standard 3" diametral pitch main gear. Only minor changes would be required to use with module gears.
3. Fully-enclosed: All the gearing can be covered to prevent wayward bits from getting sucked in.
4. Increased lightness: The housing weighs only a few ounces, thanks to more careful design and machining.
5. Minimized parts count: The housing body and cover are two single pieces that require only basic machining. The bearings and gears are all commodity parts costing from $5-$25 each.