Ah, yes, that's nice, and easy to do, to boot. I was thinking something like this, but with the possibility of tightening / locking either pivot independently.
Show Image
(http://constructionmanuals.tpub.com/14273/img/14273_179_1.jpg)
As long as you can live with a reduced mobility (±20° or so in each direction) and have access to a lathe, you should be able to make a self-contained, lockable ball joint that swallows the cable as well, something like this but drilled through :
http://www.automotioncomponents.co.uk/en/catalog/rotary/ball-socket-joints/lockable-ball-and-socket-joints/r3540#
Awesome idea! If I get access to a lathe, then I could increase the ball diameter and joint on the female end, and get more degrees of mobility.
Definitely an interesting design, and it would keep the wires internal. I know some machine shops in the area, and might be able to get joints turned out relatively inexpensively once I prove a design.