Some years back I saw a grammar based editor at Sony.
It allowed you to type a sentence in romaji and it would display
a romaji/kanji/kana representation of the sentence at the bottom
of the screen.  Since it 'understood' the sentence, by the time
you got to the end of the sentence, it had a pretty high probability
of having it right.  You could then tab over to any word and cycle
through hiragana, katakana, romaji and the possible kanji equivs.
Of course, as you fixed each word, it could be changing all of the
unfixed part to match.  I'ld hate to think how much code was
behind it.