For me Brian would have tolerated and been polite but not too impressed with Yoko and her presence in the studio. He would also have probably tried to have a nice polite word with John about what he was doing, tho it probably wouldn't have went down very well and ended up with his addition to the list of those John and Yoko thought were against their being together.
However this is all based on a time variation and who knows what would have happened if Brian hadn't died, maybe John would have remained in his Tudor mansion boredom. Change something as huge as a ...