Jumping on the 'test it first' bandwagon here. I've used MTG on all sorts of horses and have not had a reaction, but I have seen a horse who has (he was photosensitive as it was. Not sure why they used it so liberally on him without testing it first). You don't want that to be your horse. A short mane is nothing compared to blistered skin.
As for the smell...honestly, it's kind of nostalgic for me. Like kopertox and other barn smells. Doesn't bother me in the least, but you definitely can tell when someone has used it.