I will say I had great results in making a horse sound for flat riding by building it up in the Pessoa Lunging system. It helped the horse build a topline to support himself and his rider before putting a rider on him.
You can't really hide it, but if you gently massage the muscles all around it, it can help. There are different causes of hunters' bump: if it is a subluxation (dislocation) then chiropractic treatment is needed. If it is tight muscles, grooming and working on the area will help as long as your hands are soft.
Make sure you work her in good footing. No jumping if there is pain in the area or in the lumbar area. Make sure your saddle is not causing lumbar pain.
You can look on my website for more tips. Wish I had more time to help!