Why the heck I can't just Ignore All or Add an accented word to my user dictionary with a single click is beyond me.

I would guess that it's because the folks who are in charge of adding features to InDesign didn't figure that anyone was going to need a one-click way to do this. As a language-industry pro I am somewhat horrified that you overtly want to pollute your dictionary with words from multiple languages... but hey, this is your project and your working environment, so it's your call. That being said, I can think of a few potential workarounds:

1) Adding a keyboard shortcut for Edit -> Spelling -> User Dictionary.  That way, once your word is highlighted, you only need to enter two keystrokes - one for your shortcut, then Enter to add the highlighted word.

2) If you were to upgrade to a later version of InDesign, you might be able to use Adobe Configurator to create a custom panel that would have a single button to add a word to a user dictionary. Unfortunately, it seems that it's not available for CS4.

3) Put in a feature request, and wait.

Well, shucks.  In IDSC4 I, too, cannot add "Māori" to the dictionary English: USA.  I can add an ASCII word to the list in ID, export to a text file, add "Māori" there, re-import the list, and Māori works.  Not much fun, but maybe one of the scripters can diddle something.  Surely other dictionaries don't disallow diacritics.

Oh well, I guess it's good that we both have a very appropriate and correct spelling of "Maori" in our respective user dictionaries.

I tried this in both CS4 and CS5.5. Sorry for not reading your post thoroughly, Owen - in CS4 the "Add to Dictionary" does nothing. In CS5.5, it adds Māori to the dictionary, and behaves exactly as you'd expect. Haven't yet tried in CS5 or CS6.

Have you looked at importing a word list? It's a long shot, but it might work.

I'm a little surprised that you seemingly can't add an accented word to an English custom user dictionary. But I'm not too surprised - depending on your style guide of choice, accented words are loan words by definition in English. I was not able to add words that had a-with-acute to my English custom user dictionary... but if I marked the word "Bláh" as Spanish and tried to add it to a custom Spanish user dictionary, it worked just fine. My investigations into multilingual indexes in InDesign incline me to believe that, for CS4 at least, there's some kind of gatekeeper that has a list of permissible glyphs in each language, and it won't let you add words with furriner glyphs. I can add Cyrillic gibberish to the Russian dictionary... but not to the English or Spanish dictionary.