This is probably a good place to expose the naked truth. When I look under the covers of keyboard harmonicas I see one thing that surprised me. The reeds come before the air control valve. Hence, the reeds must be buried inside the instrument. I suppose this helps with tone and sound volume as the reeds are in chambers of varying designs.
The first time I saw this I was surprised as I just logically assumes the control valves would come first then the reeds. Harmonicas have the control valve first (your tongue) and then the reed. This allow the reed to be right out there in the air with no chamber around it. Pure reed tone flapping in the wind. And it is quite a beautiful and warm tone.
When the reeds are exposed to the outside of the instrument, you have a better opportunity for sound modulation.
Having the reeds on the outside is probably the reason for the lower sound volume of the harmonica.
Relating this back to melodica design: having the reeds exposed to the outside of the instrument and having them on the back of the melodica would allow great experimentation with covers, mutes, materials, hole configuration, and of course my patented belly wah.
Having the valves on the inside of the instrument, closer to the keys, would shorten the linkage and would provide finer control over articulation.