I don't think (or actually see where) anyone is looking for a theoretical or hypothetical endorsement from Brian on using AI in these discussions. The question was asking opinions if he would have used it had it been available 60-70 years ago, and in my opinion the answer would be a resounding yes, as would be the same answer if Brian were replaced by Lennon, Emerick, Townshend, Page, or any artist or audio engineer of note from the same era. How they would use it is one of the parameters, whether it would end up on released commercial material, if they would use it for demo purposes, or if they would simply tinker with it for fun...bottom line still being if they would use it or at least try it. How is that shifting the goalposts when it's a direct answer to the question which was open-ended and pure fantasy hypothetical to begin with? The actual facts and history we have of those artists from that era looking to new technology to expand their sonic toolbox and palette to create new sounds would hypothetically suggest they would of course try out new technology to further their art because they have a history of doing exactly that, and unless they were anti-tech Luddites who chose to record live to 2-track tape (or even to single-track mono disc) in eternity for whatever reason, they would try new technology if it would enhance their music.
Aside from doing the Beach Boys AI stuff as fun hypothetical scenarios, I am an active producer/songwriter in the industry. I use AI to demo songs for female artists. I'll also use it if a singer's (or my own) tone isn't quite nailing it for a demo. And it's not just me, lots of my peers are using it as well. I'd say it's totally possible BW and the likes would've used AI if it were around in the 60's. There's also such thing as "hybrid models", where for instance Brian could've combined his voice with, say, Mike's to get a punchier more nasal tone if he wanted.