You actually shouldn't need to fiddle with HP. Enough damage is dealt that characters are usually in danger of death in many "appropriate" encounters. Besides, I've rarely seen a party with a conscious cleric or druid move into another combat encounter at less than 75-90% of hp; they used up many spells restoring those hit points but they still have most or all of them when they get into the next fight. A fight that, again, deals enough damage to put the characters in danger of death.
HP and damage scale at just about the same rates for most of the game (the "sweet spot" is the main exception, where PC hp is usually about 10-25% better than enemy damage). Thus, scaled encounters are, generally speaking, equally deadly at all levels.
So, you shouldn't need to fiddle with HP.
Also, if the resting in NWN 2 is like the resting in NWN 1 then it's actually a full night's rest compressed into a very short downtime for playability reasons. Being able to have a safe night's rest one room away from a slobbering fiend is a luxury that the game gives and PCs usually lack.