The most promising option that remains, by my reading, is that there's nothing separate about the Horcrux contents for the Hat to key off of -- they effectively are Harry, or part of him.
That seems to be supported by this passage from Chapter 85:
Maybe because his dark side wasn't an imaginary voice like Hufflepuff; Harry might imagine his Hufflepuff part as wanting different things from himself, but his dark side wasn't like that. His "dark side", so far as Harry could tell, was a different way that Harry sometimes was. Right now, Harry wasn't angry; and trying to ask what "dark Harry" wanted was a phone ringing unanswered.
The idea is, crudely, that if Harry is a Horcrux, it is not because he has some distinct thing inside him, but because some part of Voldemort (part of his soul?) has "merged" with Harry.
It could be that horcruxes performed on things with brains are intensely unreliable -- so, instead of the brain being able to assert itself over any dumb matter it's bound to, like a book, it suddenly finds itself fighting for control with matter that has a 'soul' of its own. In this case, the horcrux gets trapped in the brain of an infant child, and you sort of split the difference - the horcrux is partially destroyed by years of being trapped in a baby brain, but leaves certain skills and intellect and preferences behind, integrated into Harry's brain. ...
The next discussion thread is here.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 85. The previous thread has long passed 500 comments. Comment in the 15th thread until you read chapter 85.
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. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
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: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15.
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: