Jon generally plays Suzukis. For a long time his regular model was a Suzuki Pro-v37ii. Lately his preference seems to be a Suzuki M-32C. The end pieces of the instrument in your first photo are rounded just like those of an M series Suzuki, so I’m pretty sure that’s an M-32C.
I would guess that Jon’s customized refinishings are done by a professional with an airbrush, by an artist or by someone who would do custom refinishing on, say, a motorcycle. The finish could not be baked on the plastic end pieces, but it could be on the M-32C’s metal tray.
I’ve thought about taking a melodica to a custom motorcycle detailer to see what they could (and would) do. I have had refinish work done on my BMW F-650GS, and the result was, as expected, a lovely durable finish, on plastic parts as well as on metal.
Decals are another possibility. I think you’d want a clear coating over decals.