Step one: Stand up and loudly explain how a patronus works, and what a dementor actually is, under the guise of arguing for a diffrent punisment - This will make the entire wizengamot, including the aurors controling the dementor present incapable of casting expecto patronum. Destroy the dementor before it eats anyone.
I don't think that simply telling someone is enough to take away their ability to summon a patronus, they'd have to believe you. The members of the Wizengamot don't know Harry has any noteworthy insight or intellect, and so it's likely they would not believe him unless he destroyed the dementor, thereby providing evidence that he does indeed have exceptional insight regarding the nature of dementors.
... That isnt actually a weakness in the plan, you know. He could stand up, explain the entire theory, slowly walk over to Hermonie, hand her his wand, and ask her to destroy the dementor as proof of her righteousness and disbelief would probably keep everyone (except Dumbledore. Note: Ask him to not intervene) from interfering right up until the point where she vaporizes the dementor. After which they would be unable to deny the truth of the theorem, and be fucked.
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: