The Shimano 127.5mm UN55 bottom bracket arrived today. There's enough space between the ring and the sensor now, but the magnet ring bowed on the Shimano BB shaft - seems as if the Shimano shaft is thicker than the ring was designed for. I'll probably pull it off later, shave off a bit from the ID, and remount it.
At any rate, it disappears nicely.
Also chopped up a spare battery/GPS box and test-mounted the motor controller. I have two Z910 extension cables on the way and will wire them into this assembly.
-Kurt