In the book "The Internal Power of Martial Arts" by B. K. Frantzis, the author had trained in both Aikido and Chinese martial arts, and theorized that Ba Gua (Pa Kua) and Aikido are so much alike that O Sensei must have been influenced by if not trained in bagua. I, having not trained in Ba Gua, will humbley admit that I have no idea, but I wouldn't rule out the possibility.
As far as O Sensei only having Japanese teachers, I find it hard to believe that a man as intelligent and hungry for knowledge in a different country with different martial arts wouldn't see whats out there. I wouldn't find it hard to believe that he wouldn't admit to taking instruction from a non-Japanese. Remember, O Sensei was talented, but a god he was not. In his privately circulated text "Budo" (published in 1938), he wrote "This manual is not to be shown to non-Japanese." Now, if you combine his xenophobia with a thirst for martial knowledge, the possibility of him getting instruction and just not mentioning it becomes a viable possibility. Unfortunately, I doubt any of us will know for sure.