I'm intensely familiar with the status of non-citizen US nationals. They are mostly American Samoans, maybe a few associated with the Northern Marianas. They have the same US passports, except in the observation page there is a note that they are not US citizens. Foreign governments only care about nationality, not citizenship. US citizenship seems to be a purely domestic idea. It's in the consitution, and although not prevented to do so, no US jurisdictions outside of AS allow non-citizen nationals to vote.

I've just been asked by someone something I never thought of: At US airport POEs, the line says "citizens". Can US non-citizen nationals use that line? What about "US Citizen Services" at embassies and consulates?

My immediate "guess" would be "yes" for both. At airports and embassies/consulates, there's nothing preventing them from servicing all nationals. However, "citizen" is a much clearer term (national can be an adjective). Few citizens know they are also nationals. So at both places they practically have to use signage with the word "citizen".

However, I can't seem to find anything on the internet to confirm my "guess". Not even US Embassy Apia. Does anyone have real-life accounts of these situations?