The odd thing is that the Japanese-region keyboard is obviously included in the EUR/USA firmwares: The vast majority of Japanese games which request any sort of text input will bring up the Japanese keyboard (which includes all of the different characters), so the required data is in there somewhere.
I guess my best bet at this point is asking someone with a Japanese console/emuNAND to boot it up for me and export a savegame with savedatafiler which I can then import.
Can anybody with a JPN emunand/console help me out? I'll send you the CIA.