Brian was a gentleman - I can't imagine him treating Yoko with anything but friendly respect, same way I'd imagine him behaving toward the other Beatle wives. Of course I'd have to assume he'd have much more contact with Yoko than with Cynthia/Mo/Patti, since none of those three were permanently attached to their husbands' sides.
Likewise, I don't see why Yoko would treat Brian differently from the way she treated John's other close associates (i.e. bandmates) - fairly cordial but somewhat cold and remote.