When applying your BBQ rub to the brisket don't worry about getting it on the fat. Aim for the exposed meat because this is what is going to take the flavor and this is what you are going to be eating. Seasoning the fat won't do you any good.
When I rub a brisket I do it on a large sheet of heavy duty aluminum foil (the 20 inch side stuff is the best) so I can simply wrap it up with all the rub inside and put it away in the refrigerator until I need it. The moisture from the brisket is going to turn the rub into a paste that will help it stick to the meat and help the meat absorb the flavor.
