It's funny, I've got the "Lovelines" album on right now and on the song 'Lovelines' a few of the backing vocals sound like Richard and yet the liner notes say that Karen did all the backing vocals. For a second I thought maybe Richard had mixed in some of his vocals in 1989 to give a Carpenters sound, but no, his sister was imitating him!
Funnily enough, I was listening to the Lovelines version of 'Lovelines' yesterday and was thinking how nice the backing vocals are on this (as indeed they are on most of the solo tracks). But I don't hear anything of Richard in them - it's just a gorgeous bed of multiple Karens!
Back to the subject of 'Who Do You Love', it's amazing what turns up on here - I never knew it had been released by anyone before Richard's version. I'd say it's the best track on Time (it's one of the few listenable tracks on there) and although Richard doesn't sound completely in control of the range needed to sing it, he does try to create a Beach Boys-esque sound to the backing vocals, which sort of works. But having now heard the Mark Holden version, I do think Richard's version sounds a bit timid in comparison and that the song probably needed a more assertive performance than it received.