DonM and others: I wanted a "momentary" function like that for bass inversion ("on bass,") for example if most are plain chords and there is just one "on bass" chord in the whole song. Can't believe the OS doesn't allow this to be a momentary function, so I did some research and solved it a different way...

There's a company called Midi Solutions that makes a module called a footswitch controller. You can program it to send one MIDI message on press, and a different one upon release. So, as long as vocal harmony on-off can be controlled with a MIDI message, you can buy the Midi Solutions module and obtain your preferred momentary control.