After a few sessions of soldering-and-checking, it looks good:
The yellow wires on the far right are temporary power connections; battery power enters through the contact studs in those large holes that press against the radio’s battery terminals. The cable in the lower right is the mis-color-coded USB cable that carries audio to & from the earbud & mic on the helmet. Not all the pads have components; I didn’t use all the parallel bypass cap locations because I wasn’t up for protracted self-resonance measurements.
The TinyTrak3+ cable solders into the empty DB9 footprint over on the left. I must cannibalize that from the ICOM IC-Z1A interface in Mary’s bike after the next Wouxun KG-UV3D arrives; with any luck, there’ll be a rainy day or two for that work.
The as-built schematic (clicky for more dots), which is pretty close to the original intent: