I would suggest working out the lodging / transportation prior to the trip. You could probably find a place to stay fairly easily w/o a reservation and a working Spanish vocabulary, but it may be harder to find 4wd transportation on the day of your desired departure to the hut.
We used Sr Reyes on our trip in January. He services were great and he was very responsive via emails. He did cost a little more than you could do it on your own, but it took some of the hassles out of the trip. I think we paid 140 US for two nights lodging, two dinners, two breakfasts, and 4WD transportaion to and from the hut. His website is http://www.servimont.com.mx/eng/home.html
and his email is [email protected]