1) Shimano wants ALL your money..
2) the battery belongs in the seat tube,
or how about just a Capacitor, and charge the buffer capacitor at the same time
as the headlight/taillight set gets its power, from the hub dynamo.

And perhaps its easier to unplug the hub than deal with the rotary cable.

But, Rohloff solved that a decade back, the shifting is on the other side
2 cable External box, 1 thumbwheel and it comes off.

Schmidt designed a front dropout contact based electrical connection,
now you unplug the hub dynamo just by removing the wheel.
wonder how the shift motor disconnects?