Magnesium is used to treat heart burn and gastric problems in humans. Overuse can damage humans and horses too I guess. Be careful with the stuff!
I knew a horse under magnesium, it made not a lot of difference I'm afraid to say.
Actually it can work with some horse or not. My horse was getting very nervous when we were building another field near his field (because of the noise). And I tried magnesium but also changed his diet too (took him off high energy hay and put him on low energy hay and cereal-free feed).
I noticed a real change, and did not see it doing any harm so it is worth a try.
It is better to use it on short-term.
I think you can get homeopathy stuff to you horse :)