I know that on other individual forums, that there are discussions surrounding foods, and what foods may be appropriate for a diabetic dog. I thought I would share this link. Not that they recommend any for a diabetic dog, but these may be good starting points to discuss with your vet.
http://www.gripetfoods.com/CertifiedDogFoods.htm
http://www.gripetfoods.com/CertifiedDogFoods.htm