Progress!
The cosmetic PCB I ordered from JLCPCB turned out really nicely I think, with some subtle patterning underneath the soldermask. I just need to figure out (a) the mechanical bits underneath to interact with the latches (I have some parts designed that I had JLCPCB print because the detail was too fine for my 3d printer, waiting for those to be delivered), and (b) the electrical bits.
On the electrical front, I’ve figured out that the T470s touchpad I have speaks both PS/2 (on TP4CLK/TP4DATA) and RMI4-over-SMBus (on SMB_CLK and SMB_DATA) with exciting interactions between both protocols. Although there exist “USB to HID-over-I2C” chips like the FT260, the conversion I need is from PS/2 or RMI4 to HID-over-I2C, ideally matching the I2C address of the stock touchpad (0x33) so that the FW16’s native ACPI information about the peripheral is “correct”. (The FT260 would be useful if you wanted to use the existing FW16 touchpad as an external peripheral, though.)
Anyway, just a Simple Matter Of Software, though that means I will need to hide a microcontroller somewhere to do the conversion. There’s sample code already written for the STM32, which might tip me toward using that chipset, although the code seems pretty portable. And, as usual, @Arya has been here first.