I was betting Mr. Hat-n-Cloak = Quirelmort.
Snape also seems like a wildcard possibility here - Zabini suspects him, anyway.
(OTOH, Quirrell knew Zabini was a quadruple agent, which could have intrigued him enough to want to see if he could be a quintuple...)
It might also be possible that even Eliezer hasn't decided who Mr. Hat-n-Cloak is yet--as he's mentioned previously, sometimes his fiction "writes itself." Thus, I suspect that Eliezer will not decide the character's identity until he advances the story a little more.
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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