Solved!
I've found out the offending key to have scan code 7D, which is not recognized by SharpKeys:
Show Image
(http://www.win.tue.nl/~aeb/linux/kbd/jp106-with-scancodes.jpg)
Source page: http://www.win.tue.nl/~aeb/linux/kbd/scancodes-8.html (http://www.win.tue.nl/~aeb/linux/kbd/scancodes-8.html)
Cheers ;-)
I think you need to make sure you have the proper driver as well. Japanese keyboards don't always install Japanese drivers by default. Try to install the nec japanese usb driver by hand. The scan codes should then work.