according to Pinyin socialists, 掰掰 cannot be transliterated as bye bye. It has to be bai bai. Library of Congress says so, otherwise Taiwan's tourism industry is threatened. And without tourists, we're all going to die.
And there are further complications. For all those people who pronounce 掰掰 as "beh beh," there's no pinyin equivalent. It would have to be rendered in something like IPA, and who understands IPA except for academics and linguistics geeks?
That's just silly. Of course it's transcribed baibai in HP, the same as in TP. In Wade-Giles it's paipai. Is that better?