I have vague memories of them keeping kegs back at a few of the Fran Well festivals (not the disaster of one that had about four beers last Autumn though.)
I think it was partly a decision to keep a few of the interesting ones available, so some beers would be put on Friday until they ran out, some Saturday and some Sunday. And part of it was that there was simply not enough equipment about.
Keep an eye out in the Abbots and Bierhaus as well. They usually have at least one cask on in the week leading up, and get a few for the few weeks after as well.