Hi there,
You forgot to give your post a topic! Anyway....
In some animals, the Cushing's progresses to a severe Yin deficiency. These animals also often have some demonstrable liver pathology on ultrasound and very high liver enzymes. Your patient may be one of these animals, especially if it has intractable weight loss, ravenous appetite and ravenous thirst.
Our successes in this (fortunately) small sliver of cases has come from using herbs to address the liver, and a sub-normal dose of trilostane to address the Cushing's. For the liver, we've found Ge Xia Zhu Yu Tang has been effective. Then we use the minimum dose of trilostane needed to get some weight on the dog, calm down the appetite, etc. In addition, a cooling moving Yin tonic seems a good idea (as you mentioned). For this, we've actually gone with Qing Ying Tang.
So I would suggest that as a little trio for your case and see what happens. Up until we went this route, we'd have trouble stabilizing these patients.
Steve