In reply to MattDTC:
We stayed at Camping Mischabel in July last year. I think we did pre-book as we were quite a big group, but people seemed to just turn up too. There was loads of space.
We mostly booked huts the day before by phone and it was fine, although a couple of mates did find the Weissmies hut was fully booked for Saturday night on a good forecast. We stayed at Almageller, Weissmies, Mischabel and Hohsaas huts, and got those nights discounted at the valley campsite when we showed hut receipts.