This isn't much to add, but I have a vague recollection that when Al started doing the song with Brian at their shows together in 2006/2007, it was at the behest of Brian more than Al (and of course Brian had been doing it himself in the previous few years). It may have even been Al that mentioned in some interview that it was Brian's idea for Al to do the song. Al hadn't been doing that one at his post-1998 solo shows.
Interesting. You would think Al would have had that one on his set list in regular rotation.
Just watched a YouTube clip of one of the C50 shows. Nice job by Al. Mike yells 'Keep if clean with Al Jardine' after, lol.