Apple cider vinegar, turmeric and ginger are good, natural choices for supplementing joints.
I mix the vinegar with water to make it a bit milder and give my boy a cup per day of this solution. As for turmeric and ginger - I give these ground, about 1-3 teaspoons a day with his grains. However, his issues were more minor than those of your horse, but these things surely won't harm. Just don't overdo the vinegar if your horse is prone to ulcers.
I have come a long way, to surrender my shadow to the shadow of my horse.