sensei is not an esoteric title or something like that, isn't it?
Isn't it just the japanese word for teacher? It's used in elementary school aswell as in the dojo. So there is nothing holy or royal or immortal or whatever in it.
We use this address with Japanese teachers. Or sometimes with teachers who lived in Japan for a while. But when it's used, it's used on an off the tatami.
Far from being an expert on the Japanese language...I took a beginners Japnese class some time back and the teacher was a native Japanese lady. She said that "Sensei" referred to a master of a given trade. Though generically it can be used as "teacher" it really refers to someone who is a master at their craft. Using that definition I find it hard to believe that so many can be a sensei.