Well, how about this - instead of trying to regulate resting mechanicaly (as it was in PoE 1), why not add several achievements of the form:
Finish the game while only resting once per X quests.
Provide a rest counter that the player will be able to view. Subtly encourage players to try to optimize on their rest/quest ratio.
Make players want to avoid resting as much as possible even though resting is perfectly available everywhere. Instead of them getting creative in order to circumvent any rest restrictions.
That would be a lot easier, because it doesn't really involve any changes to the core gameplay. Yet players who want such a challenge will try to win encounters efficiently in order to minimize injuries. Those who don't care... won't care.
The achievements don't even have to be especially restrictive. Their only purpose can be to draw the attention to the rest counter and the rest/quest ratio (which also should be available for easy viewing).