The author's clues are pushing in two different directions. "Taboo tradeoffs" in the title, and that Harry's Dark side is delivering the solution, implies an answer that is morally unnerving or at least cold-blooded.
The author's line about Harry shifting from seeing the Wizengamot voters as "wallpaper" to seeing individuals with agency, "PCs", and the line about remembering the laws of magical Britain, implies an answer that involves the incentives and 'rules of the game' of the other Wizengamot members besides Lucius and Dumbledore.
None of the solutions I've seen (let alone the few I've thought of) seem both Dark/taboo and social/voters-are-PCs.
Harry calling in the (nominally) Imperiused voters' debts: clever, invokes customs of magical Britain, makes the voters PCs rather than wallpaper, but not very Dark or taboo.
Harry threatens the crowd with the Dementor somehow, or browbeats Dumbledore into ruining his reputation: Dark/taboo, but doesn't invoke the customs of magical Britain or treat the other Wizengamot voters as PCs.
Is there a solution that both invokes magical Britain's laws / makes PCs of the voters and involves some alarmingly Dark/taboo move or trade by Harry?
Boy-Who-Lived marries Draco Malfoy?
Harry promises Lucius to speak under Veritaserum, that Dumbledore confesed to Harry, that he burned Narcisa. Harry is occlumens or almost occlumens, so he can beat Veritaserum, but everybody don't know it.
Lucius Malfoy has his revenge, and his son, so he let Hermione free. Dumbledore loses, and maybe everything is lost, but Hermione is free. That's taboo tradeoff.
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: