Re: Aikido "Tourism"
If you are in Sendai, I think John Stevens teaches there (at the university and aikido as well). If you PM me, I will try to get you in touch with him.
I think both hombu dojo are a good idea. Aikikai hombu may be interesting...since you do not have a yudansha passport. The aikikai folk here can probably help you with that. There are some good posts here and on aikidojournal about visiting hombu.
Your Yoshinkan instructor could probably help you with that Hombu. By any chance is your instructor affiliated with David Rubens? He's a great guy, and well respected, I'm sure he could help.