Twice that I know of - both for charity games. George Best wore a Celtic one for a charity game but only a spastic on the absolute bottom rung of pointscorery would claim he was a Celtic fan. But, you know that, I'm sure.
The super Celtic fan who wears a Gers top on more than one occassion. What is it about you lot. I would agree, I would never wear green & white hoops nevermind a celtic top. I remember an ex of mine buying a bikini that was green & white hoops. I had a laugh with her about it and she took it back the next day. And again a brother of Pendes was given a gift for his son which was a kids top that kinda looked like a Celtic top. No chance.
Yep, for charity. What is it about us? I'd say largely that we're not insular oddballs like the waste of clothing/oxygen/carbon that you are.