I thought I'd detail our excellent experience for others coming across this thread.

We flew LAX-NRT on biz class on ANA. On coming out from immigration and customs, ANA check in was to our right. We went in and asked about upgrading on our NRT-FUK flight. Seats were available, we paid our 7,620 Y per person (roughly $80) and were given our seat assignments. It turned out we were on a 737 with 2 rows up front. We then went into the ANA arrivals lounge which was just beyond the check in area. There is no ANA lounge beyond domestic security. This was a comfortable lounge with food, showers, and internet. We easily passed our time catching up online.

We left in plenty of time for our flight. We were able to take bottles of water thru security. The agent said they do not check for liquids. An agent did check my husband's bottle of water, I believe for its specific gravity to be sure it was indeed water.

The holding area for domestic flights was one big massive room with about 8 gates. There was a small snack area. You took a bus to your plane. At that point, I wished I had checked my suitcase on arrival as I'm not too keen huffing it up and down airplane stairs. Fortunately, we had a gate in FUK.

The premium seats were much better than domestic first in the US. The seats had better recline, and there was a bit of a leg rest. We were delayed an hour out of NRT due to air traffic control. No matter, it was about 3am at home. We were dead to the world til the FA woke us for landing in FUK. She handed us our box meals from the flight to take with us.

5 of the 8 seats were taken. It's always a personal matter whether to spend money on something discretionary. I was glad I did it as I valued the sleep it enabled me to get.