It would simply be bad writing to set up a mysterious and malevolent figure like Hat and Cloak and then reveal him as one of the story's established villains. It's redundant, a wasted move, to reveal that the villain was secretly a villain. It drains tension from the story to reveal that the heroes were only facing one opponent, not two. I would rule out the possibility just by assuming a competent author.
A point in favor of Hat and Cloak being Grindelwald: the playing card he chose to represent Dumbledore was the King of Hearts. ♥
Why do you think that particular Santa Claus was H & C ?
Sounded more like Lupin to me, with the 'getting into more trouble than James' reference.
(The HPMOR discussion thread after this one is here.)
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. There haven't been any chapters recently, but it looks like there are a bunch in the pipeline and the old thread is nearing 700 comments. The latest chapter as of 7th March 2012 is Ch. 77.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: