I thought about that myself. I had purchased a Suzuki Pro 37 several months ago but returned it. I now wonder whether all of the sound was projected from that hole? If so, you could probably hold it at the bottom and use your hand like a trumpet player to get the wha wha effect or mute the sound.
I am most interested in knowing if any current production melodicas have the holes on the back side of the melodica so you can mute it by holding it to your chest.
I just purchased a Yamaha P-25D (I think that is the designation) to test the waters with Yamaha. I like the sound of the pro Yamahas that I have been hearing online. I like the fact the reed plates are available and replaceable. And I am now hoping that the sound is projected from the back side.
That could be the ultimate Melodica for my application, although now that I think about, I think some have said note bending is hard on the Yamahas. I will provide my review of this instrument after I receive it and play it for a while.