Maybe I'm getting too attached to my own new solution. It does seem to me that getting Hermione to swear an Unbreakable Vow to seek vengeance on Narcissa's killer would work better.
Lucius, we think, does not know what 'Harrymort' wants. If Hermione takes the Vow then Lucius will think he has the answer: "The Dark Lord's been setting up a way to take down Dumbledore, a way that looks like the work of D's own allies." Even if D kills his Muggle-born pawn, that would look suspicious and perhaps lead to his political destruction. Then the noble, grieving-but-honest Harry Potter steps into the vacuum?
A Hermione in the hand is worth publicly backing down and 2 Unbreakable Vows in the bush.
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: