Nat

If its like most other specialties you will find they will want formal stacked shots of the winners and maybe other placegetters. There is no reason why you can't take these seperately in another area with flash when judging isnt going on. I personally would be avoiding flash photography while a dog is being judged or working out though. Simply because I would hate to feel responsible if the dog spooked.

If you can sort out a way to do the shots with a flash you might find youre able to cover the most important shots but avoid having to invest big bucks (buy or rent) in a lens "especially" for the event.