Umm isn't he called No-Face (I belive he's called Kaonashi in the Japanese version, whatever that means I don't know) because he doesn't have a face (duh) ...so there's nothing for the Noh mask to actually conceal...unlike our friendly neighbourhood sith lord who's "just a man" underneath.
Also I think what was meant by the secret identity was that they hide them selves.
Plus No-face doesn't really "speak" by himself, he just goes "ah ah". Nihilus does speak, it's just in a language we can understand.
Interesting point all the same.
*Pointless trivia* Jason Marsdn was the voice of Haku in the Amercian dub of Spirited Away, he also did some voice work for the orginal KOTOR (Dustil Onasi, Igear and Brejek I think) but not the sequel....so there is a connection, albeit an extremely geeky one "