Ask Keith Canham- he does do custom work too, but not exactly at a rabbit's pace.
You can kinda-sorta focus with the rear on a Shen-Hao, but it would be friction focus, not geared. But you're right, it is still only a double-extension camera, which eliminates the basic HZX from consideration. I can't see why it couldn't be converted to a triple, though, without too much work.
double vs. triple extension refers to maximum bellows extension. Double extension is a rough measure, where single extension is the front standard and rear standard positioned at their respective ends of the base plate of the camera. Double then would be the front standard extended out beyond the baseboard an equal amount, and triple would be the rear also extended a similar amount.