My disagreement is not with your understanding of the custom. Rather it is with the application of the custom.
Josh, many thanks for your interesting comment. I'm sure that there would have been many other splendid solutions to the situation, for example both of us ignoring it completely. Still, I was impressed by how Nishio sensei found his way around it.
He was always very careful to respect the customs in countries he visited, so it's quite likely that he did it for my sake more than his own.