I figured out a low-end way to jam the shutter release when the shutter is on B - just shove a push-pin into my homemade lensboard, right above the lever. The board is made of basswood that I painted black, and fits inside a recessed metal board, so there is no light leak when the pin is removed for the exposure. This keeps the shutter open while I focus, so I don't have to reach around and hold the lever down myself while trying to turn the focus knobs.

Ultimately I think this lens is going to end up on a 2x3 graphic of some type, where the focus can be set with a rangefinder.