I just saw this post from months ago. I have to agree with the flax and fish oil! All the shampoo ideas are great but it is just as much an inside problem. When I rescued all kinds of dogs and cats from the pound & streets that were very scraggly and missing hair I used an herb combination that grew back their hair back. ( alfalfa, dandelion and kelp) I used essential fatty acids EPA's like flax or fish oil. Many of the pet catalogs have the liquid EPA oils with amounts for dog's size & weight. It literally made the night/day difference with a dog that had dry skin and hair. I can let you know how to find the herb combination if you would would like to try it PM me.
Good Luck
Loree