I'm not sure if this is what you meant, but maybe Serenade could decrease wild pokemon encounter rate?
Essentials already has the Pokegear in it, and the Jukebox function includes a song (Lullaby) that decreases encounter rates. This effect seems to fit with the idea of Serenade (doesn't allow physical moves for a turn), and shouldn't be too difficult to program if the function is already there.
On the other hand, the effect isn't very useful if our pokegear can do that.
We also have several light moves that would make much more sense for lighting up caves than Flash does.