I have posted the following to the list. But it seems disappeared. :-) So again.
One other point that may or may not matter is that ... I'm not sure if this is the correct terminology, but the code points of the Japanese character sets are arrayed in a sparse matrix (?). Each plane is 194x194, rather than 256x256. I used to know why.
Although the plane only have 194 characters each, many Japanese fonts used by TeX were split by 256 per subfonts as that in Chinese CJK compact mode.
Right. Although there are many Chinese unicode fonts, Both unix and windows remap them to GBK or GB when used. As for Japanese in ConTeXt, I think it may be better to support the SJIS or other common used encoding, too. UTF8 is good, but it is inconvenience since I found few editor can save your file in UTF8 encoding under Windows. So I have to use convert tools to convert my files.
I am not familiar with the Japanese encodings. But I think it can be implement in ConTeXt as Chinese because many things are in the same way. Remap them according their encodings (JIS, SJIS,etc.) should work if we can solve the one problem that Japanese SJIS encoding has some one-byte characters in the range 0XA1 - 0XDF while Chinese and Korean only have two-byte characters.