Don't know if this is helpful but I have a book by Dr. Wendell Belfield (How to have a healthier dog). In his practice, he used high dose vitamin C for hip dysplasia in many puppies and adult dogs. It basically can rebuild collagen which is the intercellular cement that binds tissue, makes tendons and ligaments strong and strengthens all body structures. It also works as a natural painkiller. It is water-soluble so very safe to give. There is a schedule dose in the book which says:
smaller breeds (weighing up to 20lbs):
first 6 months 250mg
6 months to 1 year, gradually increasing to adult level 250-500mg
I don't have any experience with HD myself, but I do give my two senior dogs vitamin C and haven't had any adverse effects and they are 13 and 15 now. It sounds like it is a very safe vitamin to use and although dogs produce some vitamin C, he states that canine liver production is low and dogs experiencing stress or disease can use up vit. C at a rapid rate.