I'm pretty sure that the over production of cortisol is the reason your dog's belly is sagging, and no amount of exercise of massage will help. Because of the Cushings, your dog produces more cortisol, and the cortisol is responsible for the extra fat around your dogs stomach. Cortisol also eats away at muscle tissue, so that is attributing to your dogs sagging stomach as well.
Check back often, as others who are more knowledgable about this disease may respond.
Connie
Hi. Can I ask what form of Cushings your dog has? Is it being treated (Vetoryl is I think currently the medicine with the best track record)? Do you know whether a tumour has caused the Cushings and if so has a vet discussed any opportunity for surgery? As Cushings ordinarily presents itself in older dogs, surgery may be ill advised on those grounds. While I haven't heard of massage for the typical pot-belly appearance of dogs with Cushings, it is important not to cause further pain (the liver could well be enlarged and tender), so any such treatment should only be undertaken and/or instructed by a specialist vet who understands the particular condition of your dog. Tony