Did you see it? Our prayers have already been answered:
Added scripting DOpus.MusicGenres() method to access the list of defined music genres
You can now maintain all your genres within Opus and use them outside the metadata panel.
To feed them to your dialog, use
for (var e = new Enumerator(DOpus.MusicGenres()); !e.atEnd(); e.moveNext()) {
dlg.Control("genero").AddItem(e.item());
}