I've read your other thread in it's entirety and I think i have a conclusion for balancing resting on a larger scale in BG3.
TLDR: Limited rest could be a difficulty setting in the way Stabbey proposed it in his other thread and explained in detail in this one after me. It doesn't make any sens in the context of BG3 current design focus though.
"Whether long rests in enemy territory are allowed is a different question which my idea does not address."
"The idea of resting in specific areas isn't terrible, but because there are no respawning enemies, it would ultimately end up being a lot of backtracking to the safe area[...] ".
Blocking entirely the possibility to rest could become a no-outcome situation regardless of how you design the game. Any type of non -rest encounters should be a closed environment with a full rested party and some kind of info about what's ahead. Straight up. So that idea on a larger scale is something to throw to the bin after me in how BG series look. Only in the context of BG.
"What the different difficulty settings each do still needs to be determined, then balanced, then tested and re-balanced"
After me it's a difficulty setting on it's own. Due to a long list of reasons I thought about. While ressource management on it's own is a really good gameplay feature in general and present in all sorts of games from survival to RPG it's hard to implement without addressing the outcome of failure.
Your idea is currently a way to balance rest( a good take at it ) and a good one as long as the player succeeds. But what do we offer for failure? Walking around.Or restarting the last few fights now that we know them. That's why it could be a thing in the current version of BG3 even without polishing some of it rought ends admitting you know the fights hence why on "Hard" or "Insane" it could become a feature. Admitting it's aimed at people who accept punishment as a reward.
The above might be a bit confusing but hard and insane difficulties are the consequences of masochism and if the only thing we introduce via limiting sleep is failure(walking out of the fight) then it's a feature on it's own. A good feature, I love it, I want to suffer. But unless you build gameplay around the " walking part" then it's suffering as a reward only.
A game including resource management as a FORCED feature would have gameplay built around it. Random encounters that can happen on the road, maybe some kind of way to find new roads on the world map to get shorter travel time, getting full reward for completing the quest without killing everyone(minimalizing the number of needed resources, rewarding careful gameplay where you pay attention to what you use since you will get the same reward even if you kill everyone).
FULL REWARD FOR NOT SPENDING RESSOURCES is the most important part after me.
I think it's a bit too late for that in BG3.
An implementation of Rest Mechanic as resource management in an RPG outside of BG3? Absolutely doable. But as mentioned earlier, it requires way more incentive for NOT using them. Right now only spamming spells is fully rewarded and the game admits you have them constantly.
If " You play the game the way you want it " and "Failure is part of the fun" are what drives Larian with BG3 then the sleep mechanic could in fact be limited by default regardless of the difficulty type. Admitting NOT spending resources has some incentive for it. Currently not using all spells means: Doing the content slower, getting less exp if you decide not to engage in a fight (cause too many spells needed, and you still have your objective to accomplish).
I think to get actual gameplay value out of resting you need a title built around this very mechanic. BG2 implemented it cause lol why not. Night/day! Yey! Larian did it in BG3 did it and actually asked themselves: why in the actual fuck would we do it? And.... they added discussion with companions only in camp. And some other things happening at camp. It's a good enough way to give the camp a reason to exist.
D&D 5th edition did it and....they have a GM that will tell you what happens if you fail. Did you use all your spells after killing the 3 warlords? Well... you sneak out. You face a goblin patrol, they see you. You run away, make a distraction maybe, run up the mountains and use your last grenade to barry your enemies under rock and dirt, securing your retreat. There's a lot of things you can come up with without killing your entire party in D&D. But as a pc game? You have no idea if you're about to put your players in a situation without outcome. And you can't do that. Unless you aim at hardcore players. In which case see above.