In our many trips south going to our home in San Juanico, BCS and guiding our friends to Cabo in the winter months we have seen it done a couple of times. Not sure if it is legal.
We are returning to our home in San Juanico, BCS in July, 2012. Will check then with the federal hwy patrol and will try to post an answer when I find out.
Best of luck with your tow-S!