Maybe a cleaner way disencentize long rests is to not allow them in unlikely locations.
I too think Long Rest should be disabled in some locations. It is true that fast travel can be used to defeat such checks, but disabling the button would still present a deterrent. You can fast travel out, but it isn't always easy to get back where you left off. There is also the hope that harder difficulties will disable fast travel all together or the game will eventually introduce a toll to use the fast-travel system so that there is a cost of some sort. (Roughly equivalent to buying potions of healing for everyone).
