Got my hopes up, but it doesn't look like the recent change to Soul Reaper (DoT now damages the target if the DK dies before it expires and the target is below 35%) applies to Dancing Rune Weapon. I determined this from in game testing. We will still only get 1 extra DoT hit from a DRW cast.
There is a question though of how hard it will hit if the DK is dead. As I understand it the DoT takes your current AP/mastery values when it hits (AKA it dynamically updates). I wonder if the DoT takes your base AP (likely) or if it is now snapshotting (unlikely as it is a significant change).
As you were.