I wouldn't worry too much about it. Really, if you want to save them, then DON'T send missionaries to Japan. Apparently if they have not heard the gospel's, then they will get a free pass, just like babies do. If they do not have access or enough knowledge on the subject of Christianity, then they cannot be held responsible for not knowing the "right" path. I say, leave Japan alone and you be doing more good than trying to spread the faith at a time apparently so close to tribulation.
peace