If you don't have the proper paperwork, licenses, and mosaic covering the private parts, than you can get arrested in Japan. If your Japanese girlfriend rats you out, while you are in Japan, then you can get arrested.
Many porn companies in Japan are Yakuza controlled and they search out and destroy independents. If the Yakuza happen to catch you, they will screw you over. Either by calling the police on you or by blackmailing you and taking your Japanese girlfriend. And often people can tell if you are in Japan, by how the background, furniture, and location looks.
Be very careful not to disclose your location, use servers OUTSIDE of Japan, and use SSL and/or VPNs to upload to your servers.
I know, because I had a couple of foreign friends that did porn in Japan. They tried to go independent, and it didn't work out so well. It can be done, but you better be extremely careful.