I don't see why not, if you have the capability to write your own interface. It should be somewhat straight-forward once you pass the hurdle of your gateway receiving the SMS data. After that you can pretty much write a script that interacts with the Nest site via HTTP.

The (huge) drawback is that since you wouldn't be going through any kind of API, once their web site changes you'll have to account for those changes in your script. So it can potentially lead to a long road of cat'n'mouse.