I suggest MTG as well. At our farm we have saddle seat horses and like to grow our horses tails out as long as possible. We braid their tails then wrap them up in gaurd tex so their less likely to to loose some tail while swishing the flies away. I don't know if you could work something out with your horses forelock.
One of my geldings, Scotch, had a beautiful, thick forelock. Had. I decided to braid it one day, and when I came out to check on him, he had rubbed it all off. I've never tried MTG, but what works for me is to let it be. In a year or two his forelock will be back to what it was.