Coup de Gras?

[Tywyll]

So, I gather there are no coup de gras rules for killing Sleeping or Frozen figures, just the automatic hit mechanic. Is that right? Am I missing something?

If that is correct, what about a rule like this:
If you spend your entire round without moving, unengaged with any figure but the target, adjacent and engaged with a helpless figure, your can use your action to perform a coup de gras, killing a helpless target.