SI injuries can be curable at a young age if they haven't had a chance to fully develop. By the time the SI injury has gotten to the point of "hunter bump" there is no reversing it completely.
Acupuncture, chiro work and injections can ease the symptoms of it and lengthen the career of the animal. But eventually it will catch up and most horses will end up retired unsound at a young age.
If the bump is the degree of the picture the OP posted then there is little chance on reversing any of it. Even then the treatments to reverse it will only work if the horse has a strong conformation in the hind end. Sadly though if the hunter bump is as severe as the picture, it is likely due to conformation flaws, which means the treatments will not do much in terms of regaining soundness.
I would be interested in seeing pictures side on, from the hind and a video of the horse trotting out on pavement. I've encountered hunter bump a few times over the years. Only seen one horse come back enough to go back to competing. The horse retired unsound 5 years later though.
BB ~ 2014 Trakehner Bratty Mare ~ 1993 CSHA Em ~ 2007 Standardbred