I'm not sure this is true. Just because any 6 dan in Japan is allowed to call themselves "shihan", it doesn't mean that they do. It may be something that is culturally regulated (like so much else in Japanese culture). Does anyone know if all the 6 dans at Hombu call themselves "shihan"? My understanding is that in Japan you can make it to 6 dan without ever teaching a class, which would probably make a person think twice about declaring themselves "master teacher".
When my (Japanese) sensei received 6 dan (1996 I think?), I seem to remember he was given permission by his teacher to use the title (no formal certificate though). Granted, it's not a perfect test case because he was living and teaching in Canada instead of Japan, so he may have fallen into a grey zone with the rules. I did get the feeling that he wouldn't have busted out new business cards the week after the promotion without some kind of blessing.
Here's an interesting comment
by Yoshimitsu Yamada.
Strictly speaking, your teacher in Canda was not a shihan according to the Aikikai hombu regulations, whatever he was told. This is a symptom of the problem - the Aikikai's inclarity and inconsistency.
You're right, I should have said "any sixth dan teaching
in Japan". That doesn't remove the basic problem, though.