Narita Express basic fare covered by JR pass?

I will be travelling on the Limited Express Sunrise Seto from Okayama to Tokyo on the last day of my JR pass.

I understand that as long as I don't leave the ticket gates, the basic fare of any JR train I take is still covered by the pass even though it's the 8th day of travel (clause 6 here: http://www.japanrailpass.net/eng/en008.html).

However, I'm wondering whether I can take the Narita Express to the airport paying only the limited express supplement fee, and if so, how do I go about reserving seats?

Do I explain my situation to the NEX ticket counter when I arrive in Japan and reserve a seat then, or is there another way to reserve the seats inside the ticket gates once I arrive in Tokyo on the overnight train?

Thank you!

Unless I'm mistaken the Narita Express is behind a different set of gates. So you'll have to exit a set of gates after getting off of the train from Okayama and then enter a different set of gates for the NEX. I believe exiting that first set of gates then makes your JR Pass expired since you are on the 8th day.

The principle behind the train pass is that you need not pay if you board the train on the commencement of the journey on the 7th day, but arrival is on the 8th day. However, even if you do not exit the gates, any train that begins its run on the 8th day, the pass will no longer cover. For the Narita Express, the conductor will go around to check the passenger tickets, so if your pass is no longer valid, you will need to pay for the ride, even if you do manage to get on board without changing gates.

Thanks everyone for the helpful replies! :)

I've double checked the map of Tokyo station on the JR East website; it turns out it's possible to transfer from the Sunrise Seto to the NEX without entering/exiting a ticket gate.

Since only the limited express fare for the NEX isn't covered, I'll buy a separate limited express ticket without the basic fare when I'm in Tokyo.