The purpose of an HP threshold is so that you don't end a fight as soon as you roll initiative. I win init, you're paralyzed, and we coup de grace you. That's a) not dramatic, and b) can sabotage fun for climactic fights.
That's one of the reasons we agreed that it shouldn't be maximum hit point damage and the held enemy gets to roll a save to break the hold after every hit. Maximum damage on a Coup De Grace or Helpless enemy or character is too powerful. And it could be explained that damage is the result of hitting someone in a critical area. Even if the person is not moving, you could still swing and do little damage.
All the enchantment spells are too powerful if we don't allow for normal damage and a chance to save after every hit.
Having the enchantment spells do different things based on current hit points isn't the best way to deal with this.