Interesting idea.I think one of the interesting facets of 'HP as luck / plot armour' is that given that model they ought to be situational. That is to say, a fighter standing amongst allies, alert and ready with weapons drawn might be on full HP (let's say he has 35) but that same man, totally uninjured, hanging by his fingertips over a pool of bubbling lava could be modelled as having 1 HP.
What that's telling us is that a rock or arrow or any mishap in fate sends him plunging to his death. On the other hand, if he is helped up or manages to escape his perilous situation - he has 35 HP again!
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Personally, I think this is an interesting design idea for a game of heroic fantasy. Not one I'd expect to see in 5e (or any future D&D edition for that matter) but the idea of HP loss as translating, not into wounds, but layers of additional and imminent danger in a scene is something I think I'd have fun with.
In 4e, the closest I have got to this for NPCs is "minionisation" based on context/successful player skill roll. And also hit point loss from Intimidate checks.
For PCs I haven't really explored it very much. I don't think the game would support "minionising" a PC - instead you would vary the damage, eg by allowing a successful check from a friend/ally to reduce the damage that would otherwise be threatened by the bubbling lava.