Hello.  This is a late reply. :-)

> I've been using kakasi to render readings of kanji in the Japanese
> mail that I recieve.  However, I've often wanted something more.  I
> use mew + emacs to read my mail.  I've often wanted the ability to
> cause the kanji in the buffer to be rendered with furigana over them,
> or have some 'automatic popup' mode that gives the furigana in a popup
> window as the cursor flies over the kanji.  Is there anything at all
> like that available?

Nothing exactly possible to do your requirement comes to mind, but
EBview (http://ebview.sourceforge.net/), a GUI English->Japanese
dictionary application, can do the "automatic popup" thing.  I've
never used it, though.

Masao