That's why it needs to be phrased in such a way as to put them off guard. if they're consensus!dementors then hopefully that seed of doubt combined with McGonigal's and Albus's belief could tip the balance. I admit my own attempt to create doubt (by using "just say yes or no") was a bit amateurish. I'd imagine that dark!harry can do better. Intuitively it seems like all he'd have to do is create doubt in the non-believers and his true believers could carry it.
There's a lot of people in the room - several dozen, certainly. Three or four believers would not be sufficient - Harry's belief wasn't enough to outweigh a single half-dead crazy girl, so a 10:1 or 20:1 against ratio seems guaranteed to be ineffective.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
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: one, two, three, four, five, six, seven, eight, nine, ten.
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: