Japan has a Randonneuring club: http://www.brevet.jp/
Get in touch with them, and see about riding one of their events. If there isn't a brevet in your area, maybe you could talk to them about organizing one.
And just an additional note ... brevets come in lengths of 200 km, 300 km, 400 km, 600 km, 1000 km, and 1200 km. I believe Japan runs a Super Randonneur series (200 km, 300 km, 400 km, and 600 km) but I don't think they have any of the longer distances yet, however rumors float around from time to time that they are working on it.
One year, I would like to go there and ride one of their events ... but that probably won't happen for a year or two yet at least.
So ... here's a question for you ... do you speak Japanese? If so, then maybe you could check that site and see what the status on a 1200K randonnee is. Is the BMS 1300 going to be their long randonnee? Or maybe they've done it already (I see it has a 2007 date on it) ... is there anything there about another one? If so when?