Well, I have to look further (weak pun) into the anatomy of the ear canal. Obviously you are in a position of authority on this, but I have read of concern by health providers of this path of infection. Also, it is common knowledge how air pressure equalizes through the tube and hence there must be a path (why else would the Eustachian tube have evolved?). Also, I found a reference saying that the interior tissue of the tube is very similar to nasal passage tissue -- not mucous membrane, I guess, but certainly adjacent to it.
Not meaning to be argumentative; it just seems to me the tube clearly is a potential path for infection. How often it would actually function that way I have no idea.