Just an update:

Thanks to firstfan, I have received a test version of mscim-hangul.
This can input Korean properly. Works as good as on a computer now.
Finally, I can type Korean natively on the N900 from any app. =)
No need to copy and paste from GNU Emacs or some online site.
shift-space to switch back and forth between English and Korean input.

It's still not tidied up in terms of the gui popup during typing however. (missing icons etc)
So we will have to wait until firstfan does his thing and release it on extras-devel.