If trying to take a long rest to close to the previous long rest, then one of your companions in your party will say something like "we have rested long enough, we need to press on" and then nothing more happens (maybe even a small cut scene from time to time where an irritated shadowhart tells you that you can't keep resting or an bored astarion telling you he wants to find the occasional bandit to drain).
+1 to this (assuming you're allowed to rest after the companion speaks up against it). It discourages frequent long resting without preventing it. Lae'zel in particular should get frustrated (disapproval?) if you take too many long rests before reaching the creche. Or at the very least come talk to you and express her desire to reach the creche faster.
As for the companions and dialogs problem: Simular to the idea with comments from companions on the long rest issue, maybe they can comment on that they want to speak with you when in camp. In that way the player would know that some cool dialogues are waiting and wouldn't need to spam long rest from fear of missing out.
I've been told that when companions say that they're tired, this actually means that they have new dialogue/cutscene to share. Regardless of whether this is true, it's insufficient. If the companion cutscene doesn't require long resting (e.g., Astation's bite), then they should be able to talk about it while exploring the world and/or during short rests.