I got this question from one of our Evangelists in Japan, Hiroshi Ota at TechReady 6. I couldn't answer it on the spot but he sent an email shortly after to let me know he found the solution. Instead of having to install a custom ROM image that supports the language, there is a patch / fix to allow you to render japanese and even chinese fonts in Windows Mobile 6.

http://www.geocities.jp/asukal_trader/Localize/localizecab.htm

This page is in Japanese but you can find the links at the bottom of the page. here's the direct download link : http://hn801.hkes.com/Data/Localize/LetsJapanNo6.zip