I haven't yet found instrument keys to be an issue, either the plating on my Ramazan Kor clarinet (about as hard and inert as diamond) or the bare German silver on my old woodwinds (which tarnish visibly very fast). If it came up I might try the varnish idea.

Just occurred to me, an instrument where it might be more of an issue is the moothie, where not only is it in wet contact with your mouth, there is an opportunity for electrochemical circuits if you have dental fillings. This is apparently the reason some people have issues with lip or tongue piercings - worst case, the fillings fall out and you get mercury poisoning. You always wanted an excuse to have a gold moothie, didn't you?