The trick was you can not assign a midi patch to a track in a style which does not initially have midi data assigned to it. So the idea of moving over midi data from a track from a different style sounds like the thing to do.
Dan, you're on the money here. I have gotten it to work before, but it is totally hit-and-miss. That clue about having some MIDI data already on the track might be the clue.
Meanwhile, absolutely nothing I have read states that a user cannot add a MIDI style to a melody track. After all, that track is designed for MIDI, no?
I am beginning to think this is a programmatical issue (note how I'm careful with my terminology

)