[MikeBmusic]

I would think any 'arm' that attaches to the mic stand (or mic) is going to transmit handling noise. Also, depending on the harmonica, its usually necessary to back off the mic a bit when playing as they tend to be louder than the singing voice.

If you are a singer who has to 'kiss' the mic when singing, it will be tough to adjust with a mic in a holder, you'll need to have it bent down a bit, which means stretching down your mouth and neck and when playing.
I just played a harmonica (in a regular around-the-neck holder) last week live for the first time. It takes some patience to learn how NOT to hit the mic with the harmonica when singing, and to sing 'over' it at the mic.