Jon does backing vocals on Alice Cooper's Trash (the song from the album with the same title)..."If my love was like a lollipop, would you lick it?!" Great quote. Very cool song.

Becky mentioned Hell is Living without You (also from Trash), but as far as I remember it's just Richie playing guitar on that. It's on the same album.

I think Jon also does some backing vocals, played guitar and produced Aldo Novas "Blood on the Bricks". Good album, haven't listened to it in ages...might do it right now actually.

Richie plays on a Carmine Appice song called "Gonna Rain". I put it onto youtube...but that's playing guitar, no backings.

Don't Jon and Richie also play on some Gorky Park song? I don't remember what it was called.

And there's definitely quite a few more...but I just can't remember.