Hanin used to do that in the old days, so you're probably thinking of it. There was a setting to allowed you to set the display mode to Zhuyin while you typed in the romanization, and it was flexible -- you could use romanized Zhuyin ("MPS-2"), Yale, or Pinyin. See:
The Hanin dictionaries are still in the current Apple TC input methods (see "specialized vocabularies" in the preferences), so a vestige of Hanin remains. But what you're talking about is gone.
You could maybe look into OpenVanilla and see if they've got anything like that (I don't recall).
Otherwise, Apple sells a Zhuyin/Cangjie keyboard that is QWERTY, so you could transition to that -- there would be a learning curve but at least you could see what you're doing. OS X 10.8 and above supports the Zhuyin-ETEN keyboard layout -- since you are in Taiwan, it should be easy to buy one of those. I think a lot of people prefer that layout.
ER