I see that I'm not the only one who exhibits the Department of Redundancy Department Syndrome. :~)
A couple of thoughts come to mind.
- Look under your fingertips. Exorcise the keycaps from an old keyboard? Mine are about 5/32" height. Carve out a circle (1/4"?) containing the letter and inset it into your button. The black letter goes deep into the keycap on decent keybds and you should be able to sand/file the cap flat or domed for a better feel.
- Get one of the cheap lettering die sets from Horrible Freight. I think I've seen these as small as 1/8". Heat up the appropriate letter with a torch and burn the indentation into your wood. Fill with contrasting whatever.
Remember - you did say "Any old off-the-wall idea".
Art