23 weeks is not a bad time for flying. Seat belt goes under the belly, so I imagine it will probably be ok. I had no seatbelt issues much later on but I am normally quite slim.
Get an aisle seat! You will definitely want to get up and move around and be able to go to the bathroom.
If for some reason seats are all booked and you can't get one in advance, then try to trade when you get on the plane. I usually just explain that I'm pregnant and have to get up to pee every half hour, that usually works.
Bring a big bottle or two of water for the flight. You will get even more dehydrated on the plane than normal. You can buy expensive bottled water after security or just bring empty bottles and fill up at the water fountain. And bring snacks that you like.
Make sure you really do get up and move around a lot. Pregnant women are at more risk for clotting issues.
Also it's been a while since I've been to Japan so maybe this has changed, but if you end up taking the train, some trains (like the Shinkansen - bullet train) you buy tickets for specific seats in advance. If that is the case make sure that the tickets you get are specifically for non-smoking cars. Otherwise you will probably end up in a smoking car and a smoking train car in Japan is about as concentrated smoke as you can get.