OK my experience.
My principal instructor asked Hombu what the procedure was and from my end it went something like this.
I had to send my Aikido CV to the Hombu dojo. During the next year a Hombu Shihan observed me teach (at Summer School) and sit on a grading panel (up to Sandan). I then got a nice certificate and letter following the next Kigami Biraki ceremony.
So I guess there is some kind of quality control. As I see it if you join the club (Hombu) then you play by the rules.
Sure, but the rules are not the same for everybody.
Then the question becomes - why join a club that treats you unfairly? Are there really any benefits?