I am unfamiliar with the hachiriki outside of an Omoto/aikido context. I'd be especially interested in any examples from the shrines Takeda is said to have trained at. I agree that Omoto would have had their system well-established by the time Deguchi went to prewar China, but I am curious to know if some of the Shinto concepts were not so commonplace in Japan but well-established in the choice for a Utopian colony. Also, where would Osensei have first encountered hachiriki?
Thanks for your research and comments so far
600 years before Osensei, Kiichi Hogen
had a reputation as an expert in Onmyodo
- which was introduced to Japan even earlier than that. There's also this interesting book
that was recommended to me but that I haven't read yet.
Anyway, it's easy to see that these concepts would have been well known in Japan, especially to Morihei Ueshiba, who was educated in a Shingon temple and grew up reading classical texts.
Interestingly, Soemon Takeda was also a student of Onmyodo.
No need even to go to Omoto-kyo for a source.
It may well be, however, that the confluence of Daito-ryu In-Yo methods with Omoto-kyo beliefs had an influence on Ueshiba's conceptualization of Aikido.