I don't think the boot script can autodetect the keyboard map, as it's not a hardware issue. This is why you have to tell it what map you want to use, as not every make and layout of keyboard can be anticipated.
Japanese is usually jp106, which you can change in Linuxconf and there's probably a command to change it. Try key<tab> and see what you've got.
The keymap is installed as a gzipped file at boot time.
Why not just use the Japanese keymap - I regularly switch between the two without problems (in fact the jp106 keyboard is better for programming)