I thought the Model S already did this. Other than apparently voiding the warranty (!), I'm not aware how being out in extreme cold for a day or two could be a problem as long as the battery has enough charge to run its heater. Perhaps Tesla is being cautious so as to avoid fully discharged, frozen batteries.They should design electric vehicles so they have the capability to at least run the battery down and keep themselves warm for at least a couple of days during extreme temperatures.
The Nissan LEAF (except the very earliest ones shipped, like mine) has a battery heater to prevent the battery from freezing at -20C. As long as there's enough charge in the battery, it'll keep itself from freezing. I'm not aware of any warranty restrictions on using the LEAF in frigid climates, and I've seen reports like this one of LEAFs being used in the Arctic. More range would be nice, though...