I'm certain they could. Not sure they could reach the highs (or better, lows) of
jerry33 but I'm sure they could create a mode that improves on what we see today. Why aren't they doing it? The risk that the driving performance in THAT mode becomes what the Model S is known for.
I certainly wouldn't mind having an Eco mode on my P85D. I don't think I'd spend a lot of time in it (heck, I experience range anxiety with my 60 only a hand full of times), but on those days when every Wh counts that would sure be useful to have.