As part of Tidbit Tuesdays on Facebook, an anecdote about Trevor was posted on Varese Sarabande's official Facebook page yesterday, an entry in their ongoing Score Stories series, courtesy of cellist Armen Ksajikian.
https://www.facebook.com/varesesarabanderecords/photos/a.425284527663838.1073741838.190580891134204/479326552259635/?type=3&theater
However, regarding the timeframe of the story, the only gig I could relate this to was Yes' performance at the Hollywood Bowl on July 30th, 2001 featuring the Hollywood Bowl Orchestra as part of their Yessymphonic tour. But I have never read any other account which indicated that Trevor actually performed at the show, though I can definitely believe he was there. So if this is correct, that's another example we can add to the pile of: "You can leave Yes, but you can never really leave Yes."