On the other hand, if it is there (a highly unlikely situation) it would be immense evidence that it wasn't Dumbledore, or at the very least that it was an accident of some sort.
But this is mostly useless speculation given what we know.
I was thinking the exact opposite. If he enshrines "my closest friends, and those who died of my worst decisions," then someone he burned to death seems more likely to fit in the latter category than someone who died for other reasons. If, say, Voldemort killed Narcissa to manipulate Lucius and/or to amuse himself, then I'd expect Dumbledore to regret this death but not to rank it among those he regrets "most of all".
I did neglect the possibility that she was working for Dumbledore somehow. I probably shouldn't, since that would increase the probability of Mor!Voldemort killing her in this way.
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: