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:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
If his explanation about the nature of the patronus was believed, the active patronuses should dispel. So if he demonstrated and convincingly explained his patronus and chose not to destroy the dementor he would be the sole person in the room with control over the creature.
EDIT: The trouble is, that even if he manages to control the Wizengamot in this way, he can only control them for so long, as they remain in the room.
So he probably would have to rely on their status and/or 'Most Ancient Tradition' keeping them from fleeing the room or calling in reinforcements, while he threatens to release the dementor and/or the information.
(Note that he wouldn't have to reveal his dementor-destroying ability this way, just his 'perfect shield'.)
That would antagonize the wizengamot very, very badly. Not a good long term strategy, even if you intend to dissolve it. Destroying it, explaining, and letting them work through the logical implications on their own suffices. If most people lose the ability to halt dementors at all, and a few people gain the ability to destroy them outright, they are no longer usable tools of enforcement, and disposing of them all together becomes quite urgent.