I was wondering if we can build a list of tracks by other artists on which Cass sings backing vocals? I'd like to add 'Wasn't It You?' by 3's A Crowd on the Dunhill label (also on the Christopher's Movie Matinee LP).
3's A Crowd's lead vocalist Donna Warner is one of the few of the time in my opinion to come anywhere near rivalling Cass in terms of power. I can imagine Cass being impressed upon hearing their earlier Epic single 'Bound To Fly' and offering to produce them (with Steve Barri).
I haven't figured out which tracks(s) on the first Crosby, Stills & Nash album Cass sings on, but it might help if I had the actual LP of it. :^)