Ah, but have you tried it yet Alan? 😉
Vintage Wurlitzer electric pianos use reeds and their various service manuals advise adding a bit of either solder or beeswax to the end of the reed to lower the tone.
I’m in doubt whether that would be advisable for the melodica reeds. The Piano reeds are struck fairly strongly with a miniature felt-wrapped hammer instead of vibrating from blowing.
The fingernail polish sounds like it might be worth a try on a junker some day.