I looked at the Myers pickup again. Nice endorsement by the way. It certainly looks like a high quality microphone and preamp and perfect for many melodica applications. My issues with using it on the sound holes on the back of my Hohner Piano 26s/27s is twofold. First, I often hold the whole back of the piano harmonica against my chest to muffle the sound. I also roll the back of the keyboard harmonica against my chest to produce the “belly wah” effect. And I bang the back against my diaphragm to get a vibrato effect by pushing the air out of my lungs in pulses. So mounting a microphone on the back is not possible in my application.
In addition, I play my Shure SM57 microphone quite a bit by waving the piano harmonicas around the mic for modulation effect and moving the instrument close to and far away from the mic depending the notes I am playing. For example, when I play low notes, imitating a baritone sax, I get right up close to the mic for proximity effect and to boost the volume. When I play very high notes, I pull back from the mic so the notes are not piercing. While you can modulate volume using breadth control, playing the mic enables me to play soft but still send a strong signal (moving in close), or play wicked, powerful, high chopped chords to get the sound of power, but not overpower the system (by being far away from the mic).
I think I do have a partial solution to my monitoring problem that I will put to the test and report on soon.