As for non-Japanese; they can not become members of the Oomoto kyo. Most Shinto kannushi see Shinto as an indigenous religion. They will most of the time urge you to pursue your own indigenous religion. This idea was in a similar way also expressed in macrobiotics; only eat what in your own region can be grown.
O Sensei must have just assumed that his students were aware of there own indigenous believes.
Acrtually, that's not quite right, there are non-Japanese members, although they don't really actively encourage it anymore. There was more outreach in the past - an office in Paris before the war, for example. I think that it's clear that Onisaburo Deguchi believed that he was destined for a role on the international level.