Encounters are more fun when they are preplanned. The monsters are hand selected; the terrain is designed to match monster tactics and new challenges for the players; treasure is pre-placed, etc. Forcing the DM to continually develop ad-hoc attacks on over-rested parties leads creates the possibility for messy or uninteresting encounters. The game is more fun when the players run through the DM's pre-planned, carefully-designed encounters than random encounters. Sure, a random encounter or two is fine on occasion, but parties who nova and rest frequently are also going to force a higher than expected number of "random" encounters. I guess the DM can plan fewer "story" encounters and spend more time on the "random" encounters that he knows will happen because the party wants to nova and rest but even that is challenging because the DM does not know exactly where and when the party will decide to take their rest.
Assuming that the first half of your statement is as self evident as you propose, this does not lead to the conclusion at the end of your statement. If I build an encounter (with a map, chosen creatures, terrain effects and powers, etc.), I do not have to pick a place on the world this encounter is taking place. Wherever the party happens to be is where that piece of terrain fits. If it is an encounter in the forest, for example, and I had a field of deep grass already planned, then the party happens to come across a meadow in the forest. If it is a an ambush I had planned, and they set up camp for 17 hours, it is pretty easy to change the middle of the map to have their campsite. And, even if they somehow move so far in a day that instead of being in goblin territory they have moved into knoll territory, I can quickly reskin the goblins into knolls and use the same tactics.
But most of the time, if they are in a forest, and I plan an encounter for a forested area, they are not very likely to have moved out of that area. Especially if they have a tank who's speed is limited to, in most cases, 2.5 miles per hour. (25 miles a day is assuming they are marching, and not resting, for 10 hours).
Or, if
everything needs to be completely planned out in advance or "so be it" (implying that fun will not occur, or at least if it does it will occur at an astoundingly suboptimal level), then I will just run a railroad adventure instead of a sandbox so when they say "we set up camp" I can say "you find no suitable place to set up camp, and are now continuing into the mountains where you find..."
While I am not as against creating a mechanic for this as you seem to be against a non-planned encounter or two, and I have contributed to the creation of a functioning mechanic to resolve the problem to the satisfaction of the people who would like a mechanic for this, I think we are solving a problem with additional rules that can easily be solved using the rules that are already in place.
Given all of that, how about this option?
Until the party reaches at least one milestone, they get no experience. Then, for each milestone reached, they get 50% xp. So pushing to that 6th battle has a high reward for the high risk involved (150% xp for the day). Additionally, make sure the party knows that in order to count toward a milestone, the encounter must be a threat. In other words, only encounters of L-2 or greater count toward your milestone for the day. This last rule should prevent them from milking the system by going nova on a dragon early on, and then battling squirrels the rest of the day to get 200% xp on the dragon.