Oh, that reminds me. There was a mechanic in another game that had sacrifice HP as a mechanic... but it was also a reward, because there were number of abilities that empowered you at half or lower HP.Oh I see what you mean... hmm... well we could always have it drain Hit Dice instead?
Similar to "Bloodied" from 4e.
Could do an aura or whatever that rewarded low hp to make it work.