Actually, I remember toying with a similar idea some time ago, but I eventually forgot about it. Initially I thought of auto-use, death-preventative healing potions, but I think I could turn them into something better. I also have to make sure such a thing is effectively balanced, otherwise it really will feel like just extra health if I'm not careful.
It would work almost exactly like the Reserve Tanks in Super Metroid, in that it's an item that is filled by getting health drops even after your health is maxed. And then the item drains itself once you run out of HP. But to prevent it from just being extra health in disguise, it could apply the vanilla health potion cooldown after use, so that it has some cost to it. It could also work in that rather than refilling 100 HP per tank like in SM, it only refills 20 HP, which is 1 heart in Terraria. So, with all 4 tanks, you only get 80 HP, but it prevents an otherwise premature death if you forgot to press the quick health potion button (I could also do something so that the cooldown starts low and scales with how many Reserve Tanks you have). And maybe it doesn't stop at just 4 tanks, or something. There's a lot of balancing factors to consider.
How such an item will be implemented, let alone equipped to the player, I haven't decided, but I'll probably figure something out. It'll likely be post-hard mode or something along those lines.
(Sorry if that seemed long. My mind went on a creative spree and I wanted it written down somewhere.)