"When we were kids we always used to say, ‘Okay, whoever dies first, get a message through.’
When John died, I thought, ‘Well, maybe we’ll get a message,’ because I know he knew the deal. I haven’t had a message from John.”
Eventually the message got across. During the recording of the anthology a white peacock appeared and Paul was sure that it was John.
“I said to the other guys, ‘That’s John!’ Spooky, eh? It was like John was hanging around. We felt that all the way through the recording.” -Paul
Also later when they played the single backwards they found John again.
“We put one of those spoof backwards recordings on the end of the single for a laugh, to give all those Beatles nuts something to do. I think it was the line of a George Formby song. Then we were listening to the finished single in the studio one night, and it gets to the end, and it goes, ‘zzzwrk nggggwaaahhh jooohn lennnnnon qwwwrk.’ I swear to God. We were like, ‘It’s John. He likes it!” - Paul
So yeah, John kept his promise.