I've been cogitating on this and would love to hear from someone more immersed in Japanese culture than myself.
The question is, how would a senior Japanese teacher view this situation? I could believe that the correct, accepted behavior for a Japanese student in this situation would be to go train with whomever he wanted and not tell the teacher about it. The reason being that the teacher has made his position known; for the student to argue with the teacher about it would suggest that the student knows better or that the teacher was wrong and should make an exception; by not telling the teacher, the student acknowledges the teacher's right to make such restrictions; and that as long as no one tells anybody anything about it, everybody can go on pretending it's not happening and everyone's honor is satisfied.
Not Western morality, but consistent. Or am I making it all up?
I don't know about Japan, but I do know that here in the US there is one large prominent organization where members are discouraged from training outside of the organization. However, I know of several high level seniors in that organization that do in fact train with aikido teachers from outside their organization, as well as other arts. It's just not talked about inside the organization. And I'm fairly sure the head shihan is aware of what they're up to.
A pretty bizzare situation if you ask me. But I guess it's a do-able option if you want to actually grow yuor aikido while still remaining affiliated with that organization/shihan.