Sorry to hear about your problem it must be very frustrating. Yes I am in the lucky camp that has not seen any noticeable issue with dirt but I readily believe it is an issue for others.
I don't think Nikon's silence can accurately be taken as a customer service matter, I think it's a cultural thing. If so it's pointless being frustrated and expecting a change because it just won't happen. The Japanese executives for whom this is the purview of just don't see things the way we do in Europe & North America. I wouldn't pretend to understand Japanese culture but I've gleaned enough to know it's very different than ours where matters of pride, prestige and solving problems are concerned.
If I were you I'd just return the offending model until you get one that is satisfactory. If it effects the bottomline... that's one thing that shareholders and CEOs of all cultures understand.