New boards sent off to JLCPCB:
My previous board suffered from being designed around an FPC connector that was not in fact top-entry as digikey’s metadata claimed, nor was it bottom-entry, but in fact seemed to be a two-sided deal and thus incompatible with the actual keyboard FPC. The RP2040 part worked fine, just ended up with nothing to talk to.
The new version benefits from my actually owning a FW16 now to measure against, and should place the (different!) FPC connector in a useable position.
The second more-linear board fits under the mouse buttons and interfaces with the touchpad connector to fake out the FW16 into believing a touchpad is connected. As an extra bonus it has space for a 2x4 header so I can use it for prototyping the “ThinkPad touchpad with physical buttons” hack.