There's a few song writing contributions on Genesis albums from ex-members or people around Genesis. John Hackett had his little share of Get 'em out by Friday as well as Cuckoo Cocoon. Anthony Phillips wrote the basics of The Musical Box and Twilight Alehouse, both songs being recorded after he was no longer a member of Genesis. Mick Barnard also had his tiny share of The Musical Box. Peter Gabriel wrote the lyrics of Los Endos. And finally, Chester Thompson is credited for the Drum Duet.
Besides the Turn It On Again medley, they also performed that Supremes' song in the solo medley since Phil covered it. And in the early days they had a number of cover versions in their set lists. For the sake of mentioning it: the bootleg demos from Selling England feature them jamming 'You Really Got Me' and 'The Last Time'; and, let's not forget about Peter's regular introduction to Supper's Ready which featured a short whistled version of some folk song, I can't recall its name.
The Lamb has a number of quotations and references: The Drifters' 'On Broadway', 'Raindrops keep falling on my head', 'It's only rock'n'roll but I like it', 'Broadway Melody of 1923', 'In The Mood' (referenced in the lyrics); and indeed the story mentions "subterranean homesick dues" Rael won't have to pay.
But yea. Basically all this rather confirms your statement. Genesis never made cover versions in the real meaning.
As for guest musicians, well, David Thomas sang on FGTR, Arthur Greenslade arranged the brass and violins, and John Anthony had his moment in The Knife. And there's still that live version of Your Own Special Way with that Australian string orchestra. And those two little boys singing "We will rock you rock you little snake"...