Warren Zevon may never had his big breakthrough (although it would’ve been deserved), but he always had the respect of the whole 70s LA rock scene, that’s why members of Fleetwood Mac and the Eagles sang background and eminent session cats played on his classic albums.
“Love the One You’re With” on Steven Stills’ first solo album had both Crosby and Nash singing backup. That might have been expected. So might have been Stills’ girlfriend Rita Coolidge and her sister Priscilla. But John Sebastian is a surprise. He sang backup on three other songs on the album as well. A different three songs had Cass Elliott and Claudia Lennear doing backup.
To complete CSNY, Neil Young had Linda Ronstadt and Nicolette Larson singing backup on his American Stars ‘n’ Bars album, billed as the Bullets. Larson had sang backup on many albums before becoming a star with her version of Young’s “Lotta Love,” a song on Young’s Comes a Time album she had sung backup on.
I think you have your timeline wrong, “American Stars And Bars” was from 1977, and “Comes A Time” from 1978, though I don’t know when Nicolette Larson’s version of “Lotta Love” came out. Anyway, Neil Young also sang a duet with Emmylou Harris on “American Stars And Bars” on “Star Of Bethlehem”.
Neil Young called Emmylou Harris and Linda Ronstadt his Saddlebags 
Naomi Watts’ father (and his wife) are the voices behind some of the spoke lines on Dark Side Of The Moon.
And, not a singer, but in the Smashing Pumpkins’ Tonight Tonight, the couple you see throughout the video are Tom Kenny and his wife Jill Talley, better known as the voices of Sponge Bob and Karen, Plankton’s computer wife.
Sly Stone’s sister Rose sings backup on the Phish song “Julius,” as chronicled in a loose documentary of the recording of the 1994 album Hoist (see her at 4:00):
From Wiki.
In 1977, Larson was at Ronstadt’s Malibu home when neighbor Neil Young phoned to ask Ronstadt if she could recommend a female vocal accompanist. Ronstadt suggested Larson; she was the third person that day to mention Larson to Young. He came over to meet Larson, who recalled, “Neil ran down all the songs he had just written, about twenty of them. We sang harmonies with him and he was jazzed.”[7]
The following week Ronstadt and Larson cut their vocals for Young’s American Stars 'n Bars album at Young’s La Honda ranch – the two women were billed on the album as the Bullets – and, in November 1977, Young invited Larson to Nashville to sing on his Comes a Time album. This led to Larson’s being signed to Warner Bros. Records.
Tell it to Wiki. Can crowdsourcing be wrong?
Is Merry Clayton on Gimme Shelter a backup singer or a guest soloist?
Came here to mention her. Her vocals make that song. You can even tell Mick Jagger was impressed by the “woo!” he lets out in the background as she’s wailing “Rape and murder!”
Petula Clark and Timothy Leary were backup singers and Tommy Smothers was played guitar in John Lennon’s “Give Peace A Chance”.
I mentioned this one in a previous thread. Not a backup singer, but in the Bonzo Dog Band’s “The Intro and The Outro” when they refer to Eric Clapton playing the ukulele, it actually is Eric Clapton playing the ukulele.
At the start of her career, Cher was a backup singer on some Phil Spector produced songs, including “You’ve Lost That Loving Feeling” (Righteous Brothers) and “Be My Baby” (Ronettes).
Chaka Khan sang a very notable backup on Steve Winwood’s “Higher Love”.
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@EinsteinsHund , you are right about Rockwell and Michael Jackson being connected. Rockwell is the stage name of Kennedy Gordy, son of Motown founder Berry Gordy. Rockwell and Michael Jackson made fast friends as children when the Jackson 5 signed with Motown. Also, Michael’s brother Jermaine was married to Rockwell’s sister Hazel Gordy for fifteen years.
Jackson Browne and J.D. Souther also contributed vocals to numerous tracks on Excitable Boy.
As did Linda Ronstadt, Jennifer Warnes, and Karla Bonoff.
Of course wiki and you are right. I don’t even remember what I objected to, I must have misread your post, sorry.
Sting sings some vocals in Dire Straits’ Money for Nothing
EMM TEEE VEEEEEEEEEEEEEEEEEE
Melba Moore (as Melba Moorman) and Valerie Simpson sang backup on Blood Sweat and Tears’ first album. Both became stars not long after.
Justin Hayward (the Moody Blues) sings back up on “The Angels Cry,” the second track from Annie Haslam’s (Renaissance) self-titled solo album.
(and he wrote the song)
I have a similar example: Will Oldham sings backup on Johnny Cash’s cover of his great song “I See A Darkness” (originally released under one of his numerous monikers, Bonnie Prince Billy).
Also on Phil Collins’s “Long Way to Go.”