From some reading up, you've lost nothing by not pursuing yet. Japan's Civil Rehabilitation Act requires the funds belonging to unclaimed accounts to be preserved and eventually deposited with the court, instead of being shared amongst other creditors. There is likely an internal placeholder claim number holding the proper amount waiting for you to convincingly attach your name to it. This is the world of law not programming, so I wouldn't let forgotten passwords or closed email accounts deter me if I were you.

I'd look into it anyway - I believe a lot of people got their BTC or a certain amount of money back as part of a class action or whatever lawsuit.