Can't fault them for being off by a couple of years regarding the release of a song that wasn't theirs nearly a half century ago.  I love the Ike and Tina version of River Deep, Mountain High, but if you asked me, right now, what year it was released, I'd probably be a year or two off in my guess. 

But, it once again demonstrates the high regard Brian's contemporaries had for him.   

And you can bet that if Brian had sung just one chorus, or had a co-writing credit for just one line...or a co-producer credit....it wouldn't come in for as much criticism.

I disagree.  Just look at reviews of 15 Big Ones, MIU Album, and Keepin' the Summer Alive to see songs Brian either produced or co-wrote getting the same critical treatment (or worse) as Kokomo.  It's Carl and Dennis that are generally regarded as the saving graces of the 1978-80 albums, not Brian.  GIOMH didn't exactly get glowing reviews, either.

That said, how a Brian association might have helped the song's reputation...

Suppose BW88 had had the group do harmonies, therefore being a Beach Boys album (titled Love and Mercy) instead of a solo one, with Kokomo in place of One For The Boys.  It would have been the big hit on an album with Love and Mercy, Melt Away, Night Time, and Rio Grande.  Being on a very good album rather than the... not so good one it was on would have made it part of a creative revival for the band, instead of representing the era where the Beach Boys became a joke. 

It would still be considered cheesy fluff with a great Carl vocal, but it would be seen in a completely different context.