Well I have some doubts. But I agree it was quite possible even likely. In fact I believe preliminary plans were discussed, with both Paul & John agreeing to play together with Ringo, on the album that became Stop & Smell The Roses, with the three of them doing Nobody Told Me & possibly Life Begins At 40. I believe Lennon also said, to Jack Douglas & others, that he thought Harrison would get on board to reunite the Beatles playing behind Ringo, altogether.
I believe ( double check me on this part ) Lennon also said, he thought the initial ruse or plan to back Ringo, would then lead to other recordings, temporarily reuniting the Beatles.
Besides all that, Lennon I believe signed some contracts, not long before he died, saying the Beatles intended to reunite with new music, for "The Long & Winding Road" a documentary later retitled as we know it-The Beatles Anthology..wow huh
These contracts I believe only came to public light, when they were found after Lennon's death. I believe all 4 Beatles had long signed off, on a Documentary as told by the band, Neil Aspinall had spoken of this & assembled hours worth of footage & dialogue. Some of which was actually used in Anthology.
Although Anthology hit tv screens & record stores in 1995, I believe some of the interviews done by individual Beatles for Anthology, were done as early as the mid 80s,. Just several years after Lennon died. But again Anthology began in the early 70s, spearheaded by Aspinall.
They nearly reunited in March 73, except apparently McCartney had visa problems getting into the USA in 73, & Lennon was afraid of being denied re-entry if he flew to England & tried to come back in 73.
Had they wanted to, McCartney could have overdubbed, to a slave-copy reel of I'm The Greatest in England & airmailed it back to Richard Perry. Or built a segue of 16-32 bars between I'm The Greatest & Six O'clock. Harrison could've added guitar later, he lived in England like Paul anyway.
Having said all that. Any one of them could have changed their mind before it happened. But I think all the evidence indicates the documentary was 99% & the backing of Ringo was at least 75-85% from what Jack Douglas, May Pang & Mccartney say about it. George & Paul did back Ringo on 6 tracks of that album. But not together, George produced 2 songs & McCartney 4, actually 5. One was bootlegged "Can't Fight Lightning" where Paul drummed & Ringo played guitar.
Oh yeah the John-George feud over his book was blown out of proportion I agree, easy enough for that to blow over on it's own or with one phone call.
Harrison played on Instant Karma, The 5 Imagine tracks, & did Cold Turkey live with John before Lennon moved to New York. And George helped on I'm The Greatest. Hands down the 5 Imagine tracks were the main post break up collaboration between the two. It would been nice if John & George had done some Beatley oohs & aahs on Greatest. Perhaps without McCartney there to do the high parts, just John & George alone didn't think they'd get a great blend just the two of them.