The British singer, 63, who famously performed at the 1997 funeral of William's mother, Princess Diana, and at her memorial concert a decade later, was asked about the wedding on BBC Radio.
He replied that he'll "probably" perform – but quickly quipped, "I'll probably be outside busking."
Next Wednesday, Dec. 1, for World AIDS Day, John will be guest-editing the British newspaper the Independent, but he told BBC Radio 2's Steve Wright not to expect too much royal-wedding coverage. "I think we've had enough of that for the time being," he said.
Meanwhile, William and Kate were at the Buckingham Palace together on Thursday holding meetings with their aids. Palace officials say invitations have yet to go out for the April 29 wedding, and they won't comment on possible big names on the guest list.
"It is all speculation," a palace spokeswoman tells PEOPLE. "Nothing has been determined. We are still in discussions. They are only now sitting down and starting to think about things."
The guest list is likely to include ordinary people from charities associated with the couple and members of the armed forces.
A report in the Daily Mail suggests that Diana's brother, Earl Spencer, will have a key role at the wedding, perhaps even making a speech, but the palace rep would not confirm that.