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 firstname.lastname@example.org.
I agree, the $140 is a little expensive for Mexican standards, but it is a pretty cool place to stay. The food is great, and the Reyes family does everything they can to make the trip run smoothly. You can certainly find a less expensive place to stay, but we decided on Sr Reyes due to his tradition of excellence and we had yet to "splurge" on our 10 day Mexico trip.