Consider adapting the Energy Reserve idea to HP instead of to FP. A "Vitality Reserve" should do what you want. To fully parallel the behavior of ER, it should be depleted independently of HP, not be subject to the ill effects of HP loss, heal in parallel with HP, and heal irrespective of rest. So VR would amount to HP that you can lose without shock, knockdown, knockout, or death, and that regenerate at the rate of 1 HP/day on a successful HT roll, regardless of conditions. Most of these advantages parallel Ablative DR (which is also independent of HP and not subject to the ill effects of HP loss). Moreover, neither affects what happens in slams and falls. The differences:
Advantages of Ablative DR:
- Stops damage, not injury; e.g., 1 point prevents 2 HP of injury from an impaling wound to the torso, 4 HP of injury from a skull hit, etc.
Advantages of Vitality Reserve:
- Soaks all injury, including that from disease, poison, and radiation; extreme dehydration and starvation; critical failures that result in direct injury; vampiric leeching; and Cosmic attacks that ignore DR.
- Can be spent like HP for any ability that allows this, notably spells.
I'd say that justifies simply pricing VR like HP, much as ER is priced like FP.
- https://web.archive.org/web/20160815114845/http://www.ravensnpennies.com/2016/08/gurps101-injury-resistance.html mentioned it, page is no longer up. Raven used it to model VR as "Injury Resistance, Ablative"