I use Japanese intensely and have Japanese input on the N900, however I do it like I do Japanese input on my notebook, not bothering to install a Japanese IME, instead write Japanese in GNU Emacs, than copy/paste it wherever else.
...I've even compiled Prime, a predictive input system for Japanese, ...I'll be posting info on how to setup GNU Emacs with Prime on the N900 at some later point in time, so will drop a note here if you're interested.