My friend who raises halter horses uses Fat Cat by Vapco for a supplement. Their horses also spend a good amount of time inclined on the treadmill. I've never gone to that extent as I know my shorter, bulldog type foundation bred horses won't win halter against the 16'2" beefcake halter horses, but they can be fitted up, muscled & competitive. Hill work is great for building muscle as well as backing. Ever checked out the hindquarters on roping horses? It's from all the backing & holding a cow. Anything you can do to have him moving & engaging those muscles will help. Walking & trotting in hand over ground poles so he has to pick up his feet, etc. Also the best tip for halter horse muscle, good vigorous grooming. Use tons of elbow grease & brush him as hard as he can tolerate in short strokes at a quick pace. It stimulates the surface muscles & will tone them.
Be sure when you set him up not to stretch him out, keep his hind legs under him to show off his hind end. Be sure your halter is fitted snug under his jaw & around his nose. A saggy halter can make the prettiest headed horse look awful.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.