Okay, I don't really think this is how it'll go down - slightly too Dark Lordish. But the image was amusing, so here goes:
"It just happens that if Hermione doesn't walk, everyone but me will lose the ability to cast Patronus. Don't buy it? Oh, well, I'll just explain it to Hermione and she'll be able to testify under Veritaserum that I can do it."
Or, you know, have Hermione figure it out herself from Harry's note and do the blackmail herself.
Anyway, the blackmail potential for this is rather great, and I'd not be surprised to see it used in a more dire situation with more than Hermione on the line.
What's the in-story justification for the dementor's presence anyway? I thought it seemed awfully convenient in case Harry decided to demonstrate his Patronus 2.0 but I couldn't figure out how it'd help enough.
I'd forgotten about the potential for ruining others' patronuses, though. That makes a lot more sense, especially considering he'd just reached into his dark side - possibly deeper than he'd ever willingly done before.
My guess: it wouldn't be enough at this point to just demonstrate a superior patronus or tell people about the possibility of ruining ...
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: