Sarah, thank you for the great information on ascorbate and the impact on hip dysplasia. My husband and I have always had Great Danes and both dysplasia and knee weakness (requiring TPLO) has been a common problem. The conventional wisdom is that dogs make their own Vitamin C so there is no need to supplement.
Since we don't live in a perfect world of perfect nutrition for our dogs, I can see how they may not be making enough of their own to support the rapid growth in large and giant breeds. So then the question is how much to give, what form, and for how long? Even though it's water soluble, vitamin C in excess can have side effects in humans, and I assume there could be similar issues in dogs.
Or maybe even better, what sort of nutrition is required to give the pup all it needs so that supplementation isn't necessary? Raw vs. cooked meat, carbohydrate types (if any), etc. I don't have those answers so this could be a great discussion.