The Boy With the Thorn in his Side was originally called The Boy With the Bum For a Face, and it was about me. I had to send the bumfaced lads round to Morrissey's house to 'persuade' him to change the lyrics.
Lemmy bought me a bottle of Newcastle Brown Ale at a Motorhead gig in Middlesbrough Rock Garden - i returned the compliment with a Jack Daniels & Coke at the Moonlight Club in Hampstead London, some 2 years later at a Modern English gig.
however back to the original thought by the OP - In 1971, john Peel played pretend Mandolin for Rod Stewart's hit single Maggie May on TOTP, and got a performers fee rather than a presenters fee.
In the mid-80s, Alison Moyet co-wrote some songs with a 'Jean Guiot', including the hit 'Is This Love'. Mr Guiot was in fact Dave Stewart of Eurythmics.
Similarly, Billy Bragg recorded Walk Away Renee, which entailed him reciting a piece of prose over the top of an acoustic version of the Four Tops song, exquisitely played by 'Duane Tremolo' - which was in fact Johnny Marr.
There's a rendition of a John Cage tune called Organ/ASLSP that's underway at the moment that will have lasted 639 years when it finishes in 2640. The last chord change was on February the 5th this year with the next one planned for the same day in 2024.