Also, about scaling. If you compare temporary max HP loss to level drain (let's say temporary level drain), in DDN a single level as a fighter is what, +1/4 to your attack? or +1/3? But losing 5HP is like losing a hit die (except Con bonus) worth of XP. I think the flatter math supports HP loss vs level loss as energy drain mechanic well, compared to earlier eds.
I'd also agree with a Con save the next morning, one per HP, to see how many you get back. So if you're down 10HP that's ten rolls, you pass maybe 5-6, so still not at full. That's a tough dungeon. Remember losing your max HP makes it very much like being a lower level in DDN, since everything is so damage focused. Having a lower maximum makes even a common dire rat more of a threat, since they can actually still hit you no matter your level.