Well, I can certainly feel your pain. My boy is black, and looks fairly bay at this point. In July, he looked rather seal brown, actually.
Personally, I'm not too worried about it. With the coming winter, it will darken up, and Quic Black shampoo helps immensely for us.
Please do not use human hair dye - it's harsh and could cause serious reactions. Not to mention it fries the hair.
I plan on using a temporary dye spray for my boy's tail for our show in October, simply because it's fairly burnt looking and yucky, but I'm just going to Quic Black his body and if it is still bay-esque, oh well. Dying could make things turn out a lot worse.
I have heard mayonnaise *prevents* bleaching, but I have not personally tested this. Although your horse would smell like poo.
In a nut shell, here is my advice out of all my rambling: It's already so close to the fall/winter, don't stress about it now. Use Quic Black and forget it. Next spring, around March, try Black As Knight, research try the mayonnaise, and use the Quic Black every couple of weeks, and all those things combined should help you out, but at this point, it's just not worth the trouble to try feeding BAK and fooling around with mayo. And it's never worth using human hair dye!
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers