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.
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