After discussing this with Buffalo's US and Japan corporate offices, I have a question and some more info for you.
How did you update the unit with the Japanese firmware in the first place?
Since you live in Japan, here's some info I got from my Japanese counterparts:
It is illegal to use an overseas wireless LAN unit by Japanese law. See the following link:
http://www.tele.soumu.go.jp/e/monitoring_qa/index.htm
The only possible way that I can see to avoid legal trouble (this is NOT legal advice) is to leave the Japanese firmware on the device. As far as I know, the US and Japanese versions of WHR-G300N have the same hardware, so the firmware difference should be the only thing that has potential legal issues. The US and Japan have different regulations when it comes to this type of equipment.