I believe no one has made this in the past because it would be unreliable. "Hard" doesn't come close to it.
There is no way to get this information, so the addon would just be guessing. It could think that there are only 3 combo points on you and you would think that you have enough time before the finishing move to get a heal off, but... Really it was 5 and in a split second you're dead. WTF happened? It was only 3 combo points, how did they do so much damage? Relying on an addon that doesn't know what's really going on isn't practical.