When a Yamaha P-37D is removed from its case, the keyboard is still attached and provides the seal for the air chamber. So the keyboard is part of the inner melodica, not part of the case. There is at least one melodica whose reed chamber is removable as a unit, the Suzuki MA-32 Super II. When you remove that cartridge (the “true” inner melodica?) it is not air tight, since the holes that are normally covered by key pads until being opened when the key is depressed are all open. If you cover all holes except the one for middle C, and blow air into the chamber, middle C will sound.
I’m theorizing now, not having experimented.
So, in theory, if you built a melodica around the MA-32 reed chamber, any mechanisms and key configurations that open and close the appropriate reed chamber openings should be able to sound the reeds. But it seems like doing this with a Janko keyboard would be horrendously complicated.