I can't help you with Word but you could have a look at LightWay Text and iText Pro '09. Both support furigana and have various other Japanese features such as top to bottom right to left text, Japanese manuscript layouts, and emphasis by dots over characters.
They certainly have all the power you need for most normal writing needs. (Word is overkill for most uses.)
http://homepage.mac.com/lightway/LightW ... d_file.htm
http://homepage.mac.com/lightway/iText/ ... d_file.htm
Their english web site is a bit 1990's but don't let that put you off ! (The Japanese site is more restrained.)
Lightway Text is shareware at a very reasonable $25. I can't remember what limitations there are in an unlicensed copy, maybe none or maybe it only works for 30 days.
The emails I have had from the programmers have been very friendly and helpful so I encourage people to pay this small fee. The licence is good for all their products and all updates.
iText Pro is $15
Lightway Text is probably the more powerful program.
It has a couple of quirks however, (I think because of ShiftJIS encoding being used within the program, although it'll read and save utf8).
You must use a Japanese font to display Japanese. With Helvetica for instance you get mojibake most of the time but Hiragino is fine. Japanese file names don't display properly within the program menus so it's best to save using romaji file names.
iTextPro doesn't have these quirks. You might need to play with line spacing to get clear furigana though. The J Manuscript example in help is a good place to start.