Awesome picture Daren. Tatu, I have used and have recommended the Myers pick for a while now (see website below) It works great on my Vibrandoneon, Hohner Professional and Solist melodicas. It is an external pick up with a miniature goose neck. I run my Meyers pick up from my melodica to a pedal board to a Fender Delux Reverb tube amp as a norm. But also run several other types of amps with this pick up with positive results. speak to Greg Meyers and let him know what your application is and I am sure he can suggest the right pick up for you. It is small like you button harmonica. Uses small flat batteries and it comes with a built in volume control with no special installation required. Mention to Greg about a low Impedance converter as well for line boost. You mentioned using an accordion pick up, you are correct about the corrosion as I have tried one before. I had a little phasing issue around 7k (cancelation/drop) because where I had attached the pick up to the Vibrandoneon. Once I installed a small piece of foam to the body and separated the pick ups farther apart from each other on the foam the phasing issue was gone but the sound of the Vibrandoneon changed to my dislike. This may not be the same issue with the button harmonica. I would be concerned about placing your mouth on a metal mouth piece with a high impedance signal and moisture.
I hope this helps