Are you wanting to trigger a vocal harmonizer from the midi?
If so, you can record a track with the proper chord changes or notes and leave the volume at zero. Then set your harmonizer to monitor that track. Or you can merely set the harmonizer to read a track that already has the proper chords in it.
Most harmonizers can read either chords or notes. For precise harmonies use the note mode and record each harmony note on an otherwise-unused track. Again, set the volume of the track to zero, and set the harmonizer to that track.
I haven't done it, because it's more time than I want to spend, but it should work.
DonM