Mick, I have the impression that the out-of-focus areas in Nick Brandt's pictures are often created by the home-made tilt adapter he describes in the thread I linked before. Therefore you cannot really judge the object distance from these areas.
The Pentax lenses are only corrected to cover 6x7. If he uses tilt, this must lead to some dark corners. I think you can detect these, for example in "Lion Before Storm II- Sitting Profile".
By the way, for those who want to know more about the images on display before they make up their mind to visit the exhibition, I found a website, which more or less shows the pictures on display. But this is no substitute for seeing the large prints in person.
Chris, I am not sure you can upload images here.