overqualified backing vocalists

singers who sang backup on covers of their own songs:

Dobie Gray, on Uncle Kracker's cover of 'Drift Away'
Dolly Parton, on Mindy Smith's cover of 'Jolene'

Pete Townshend sings backup (but doesn't play guitar, for some reason) on the Ramones' cover of "Substitute."

― Montgomery Burns' Jazz (Tarfumes The Escape Goat), Friday, November 30, 2018 10:56 AM (one year ago)

Does this count?

"The Longest Time" is a doo-wop single by Billy Joel. The song was released as a single in 1984 as the fourth single from the 1983 album An Innocent Man. Following the theme of the album in paying tribute to Joel's musical influences, the song is presented in the style of Frankie Lymon and the Teenagers. It reached number 14 on the Billboard Hot 100 and number 1 on Billboard's Adult Contemporary chart. In the United Kingdom the song reached number 25 on the UK Singles Chart.

The song features Joel on lead vocals and all backing vocals. Two musical instruments are present in the song: a bass guitar and a snare drum being played with brushes. When the song is covered by vocal groups, the bass part is typically sung. All other sounds in the song are Joel's vocals, along with percussive sounds such as finger snaps and hand claps. Phil Ramone and Joel had intended to feature a vocal group but Joel recorded each of the parts himself.

I knew Angel Olsen sang backup for Will Oldham for a bit but didn't realize for a long time that she's in the video for the second version of "I See A Darkness"
https://www.youtube.com/watch?v=4iV4NwSbscg

now in her band she has Heather McEntire of Mount Moriah, who's got a great Dolly Parton-y voice herself
https://www.youtube.com/watch?v=2J7E5hqjX0o

Hardly proper "backing vocals" given that she sings much more than the billed artist, but: Annie Lennox on the 1984 solo single "Darling Don't Leave Me" by Robert Görl of DAF:

https://www.youtube.com/watch?v=M95Dws35cKQ

Lindsay Pagano - So Bad (ft. Paul McCartney)

https://www.youtube.com/watch?v=RQgLrQRaXIY

a literal nobody who was best known for appearing in an AOL commercial in 2001

Pagano was recording the album at the same Los Angeles studio where McCartney was recording Driving Rain when he introduced himself and asked to hear some of her music.

"I played him some, and he was dancing all around the studio," Pagano recalled. "Everyone was freaking out that it was Paul McCartney, and I would be like, 'Oh, I just saw Paul. Whatever.' He just became a regular person. He asked how the album was doing one day, and I said, 'Well, I'm just looking for a really simple acoustic song for the end of the album, just to end it.' And he said, 'Well, I wrote this song called 'So Bad,' which I think would go really good with your voice.' I laid down a rough track, and when I came back to work on it again, I hear this vocal in the back, and it's Paul's vocal. He was singing on it as kind of a surprise. So it became a duet."

that aol commercial song could have been legitimately great if they had just redone the entire song other than the chorus

not sure if this sort of thing counts, but two 90s hiphop tracks immediately came to mind:

inspectah deck on 2pac's 'got my mind made up' and busta rhymes on capone n noriega's 'driver's seat.'

both were cases where the "featured" mc had a guest verse that was later cut and all that remained in the final mix were their adlibbed parts. like i said, kind of a grey area.

Not quite on topic but I was revisiting Lou's "Street Hassle" the other day and was startled I'd forgotten the uncredited cameo verse by the Boss.

Stevie Nicks and Lindsay Buckingham on "Magnet and Steel"

xp Bruce also does the almost inaudible backing vocals on John Prine's "Take a Look at My Heart."

...and that's also Bruce, very audible, on Graham Parker's "Endless Night"

Q-Tip, Slick Rick, and Biz Markie back up Jay-Z on “Girls, Girls, Girls”

A Tom Waits song is the only possible context where Keith Richards would count as an "overqualified" backing vocalist.

https://www.youtube.com/watch?v=UgG4rxq8DhY

David Crosby on "Another Day in Paradise" (phil colins)

James Taylor on "Back in the high life" (steve winwood)

Shannon Hoon on "Dont Cry" (gnr)

Liz Phair on "Work" (jimmy eat world)

Liz Phair on "Soak Up The Sun" by Sheryl Crow.

apparently michele phillips and diane warren harmonize on the chorus of “heaven is a place on earth”

Billy Ocean on Scott Walker’s Track Three

Billy Ocean sings the high harmony for about 90% of the song, to be fair it's practically a duet.