Can the vow enforce factual claims about the past? E.g., "I vow that I am not Voldemort as you suspect", "I vow that I was always Draco's friend and am not to blame for the assassination attempt", etc. If yes - that is, Harry would be unable to Vow falsehoods - then he could convince Lucius of his goodwill. OTOH, if he actually tried to vow "I am not Voldemort", the result should be.. .educational.
Should be able to, since enforcing honesty would seem to be on offer if ordinary Veritaserum can do as much... Now, the question is, does an Unbreakable Vow to tell the truth overcome obliviation/memory charming/pensieves etc? One might expect powerful sacrificial magic to be able to do that, but then again, if it did, you'd expect officials of some stripe to have such Vows as matters of course and we don't see that (on the gripping hand, wizarding society is not that efficient or imaginative).
I would expect a Vow only binds you to tell the truth as you know it at that moment. Nevertheless:
"I vow that to the best of my knowledge in the past XXX. I also vow that if I ever discover evidence that this is false and I had been Obliviated or Memory Charmed to enable me to make this vow today, I will come tell you all about it and submit to your judgement with a specified possible penalty."
So you can at least bind yourself irrevocably to your new position.
...if it did, you'd expect officials of some stripe to have such Vows as matters of cours
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: