Assuming you come from a country that requires a visa to enter Japan, it is ultimately the immigration agent's decision to allow you to enter Japan. But I believe that you should ask the staff of your airline to support your application for a shore pass or transit visa to be issued on the spot.
So, in the event that you are delayed, I would ask the cabin services director if he or she has the necessary paperwork. Such a situation is clearly the type of situation where agents have discretion to allow you to enter the country - They will want to see that you are booked on a flight out of Japan. I guess they might also want to see that you have sufficient funds to pay for your hotel night in Japan.

AFAIK, there is no airside hotel at NRT.