Given that there's just more room in the 16", I wonder how hard it would be to just build an adapter board (and some 3d printed scaffolding) to just mount an actual lenovo thinkpad replacement part...

When it comes to the keyboard, the room that matters is the height, and both the FW 13 or 16 have a maximum height of 3.8mm

Unless one can come up with a way to produce a custom keyboard with the same height restriction, using something like a Thinkpad keyboard which is considerably taller would essentially require a custom taller chassis.

Not impossible, but relatively hard I would say.