A solution of potassium permanganate is what taxidermists use...It's a purple solid that mixes into a purple liquid..it is painted onto the antlers and wiped off after about 5 or 10 minutes. It turns a rich brown color and can be feathered.

Gotta be a taxidermist or a chemist to get it though.

In lieu of permanganate I'd use the stains mentioned...