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Yeah I thought about that method.

The internal button cap cavity measurement is around 11.6mm x 1.25mm with the central post removed on my version but I don’t think that size magnet would work too well through the 0.6mm steel, maybe an 11mm x 2mm N52 would do there but using the original rubber boot would be a problem, you would need to play around with some thin silicone sheet to maintain some waterproofing and maybe make a new small nubbin to get the switch working nicely…

The tail cap button is an awkward size, measuring 14.6mm x 0.35mm at the widest point (retaining lip)

The button part is then 13.88mm x 1.4mm.

So if you had maybe a 14mm x 0.5mm or 1mm magnet JB welded onto a 13mm x 1.5mm magnet that could work with some modification.
Best to file off the nickel coating and clean thoroughly the sides that you’re gluing too.

Something like this:

One good thing about the FW3A is the switch is so easy to access you can play around with different ideas no problem.