Thanks Peter -
It would be intriguing to contact/visit the various shrines where he was employed - I wonder what records remain.
Well, I have looked at the website of Aizu-Wakamatsu, for example, and the city is clearly interested in its history. So, I suspect that a disinterested researcher who could handle the Japanese would be able to gain access to families with histories.
Similarly with Tanabe in 2008. I met quite a number of people, unconnected with the Aikikai, whose older relatives knew and remembered O Sensei.
Yet another retirement project...