Option 1 - Relative search (with lanji support, obviously), using a sequence of at least 4 characters (ideally 6).
Option 2 - SJIS Dump is a great tool. Use it!
Option 3 - Look for ASCII inside the ROM. If you find some, chances are you'll find some Japanese close by.
Option 4 - Use an emulator and (painfully) watch code being executed until you find text. Probably starting from memory read of the font graphics (since you seem to know where it is?)
(A lot more options are bound to exist...)
All of the above require a good reading of the "Getting Started" section on this site. Have you read it?