Sirius Black fits Santa as well, if not better, though most of what we know about Sirius comes from Cannon.
In Cannon Dumbledore had possession of The Invisibility Cloak when the Potters were killed, but he didn't necessarily keep it. This wasn't mentioned in books 1-5 though, so EY is probably unaware of that detail.
James' best friend Sirius knows James' misdeeds better than anyone besides James himself, and is much more likely to speak proudly of them than Dumbledore.
The Portkey doesn't have to have been made by Santa - it could have been made by anyone at the Salem Witches' Institute. And it doesn't work on the Hogwarts grounds.
Santa Claus claimed Dumbledore would keep The Invisibility Cloak if he ever got his hands on it, which was completely wrong, and a rather insane thing to say if it's Dumbledore (which doesn't prevent it from being Dumbledore, but still...)
Sirius wouldn't know anything about Quirrell. Dumbledore presumably researched him before offering him the DA job. Neither would trust him.
As Harry's godfather, Sirius would care a great deal about Harry.
There is also the Meta level to consider.
Sirius Black is a major fixture of Harry Potter canon. He has not yet been featured or brushed off yet (eg the Weasleys). Every other major player from the first books has been given screentime so far. It seems incredibly unlikely for Sirius to make it to chapter 78 without a showing in one form or another.
(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: