I feel HP-as-a-resource can work, but not as part of normal play. It has to be something special. Say as an extreme - and not really serious - example, consider this version of Oblation:

- You lose 50% of your current HP.
- You gain 200% of the lost health as a shield, independent of target.

While losing 50% health feels crazy, at the same time you essentially go to 150% health if you include mitigation, which also feels crazy as a bump. And considering how much more healing there is than damage in the game right now, even 50% lost health so long as you cannot be immediately threatened (you cannot, with that shield) doesn't really account for anything. But it would not work as something that happens constantly in small bumps because only two jobs - the shield healers - cause constant low-healing on the tank and would hence automatically gain free micro-shields on their tank for no cost, while the other two healers would lose a GCD here or there for this effect and hence would probably not even engage with this. As a "burst" it works better because then you're expecting active attention either way.