Well, Framework are determined to make my life easier for me. Turns out that:

a) It's possible to make an input module as a single PCB - I was thinking I'd need some combination of overlapping PCBs and/or some 3D-printed parts. Maybe that will yield a better aesthetic result in future, but as a first cut, that's ideal.

b) The "magnetic attachment" is a solved problem - there are solderable threaded inserts which are ferromagnetic, and will stick to the magnets in the laptop. Framework have even provided a part number: Keystone 24929, and they're available from Mouser.

c) Framework have produced a reference design, in Kicad (which is my preferred PCB CAD tool), with an RP2040 included. So I can use this as a starting point for my design.

d) Someone has already created a Kicad PCB footprint for the one-key module

This will give me a great head-start. Next step: modify the reference PCB.