I'm in the same predicament with my J flight next year on this route. All the window seats appear on the site to be blocked but according to expertflyer there should be some availability (some are blocked but not booked). Are they usually not reserved for award flights or is there a date they get released? I've checked on multiple occasions and still no availability for the windows.
when I took this flight toTokyo from Melbourne in May 2018, I just kept checking the seats daily. Closer to departure the window seats opened up. They must block them out for more important JAL J types until the last minute. Our J flight to Tokyo only had 5 people - the rest was empty. Saying that, my folks took thissame flight JL773 or JL774 last month and it was choc a bloc!