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?