Dry Rub: Mix first 5 ingredients in small bowl to blend. Transfer 1 tablespoon dry rub to another small bowl and reserve for mop. Spread remaining dry rub all over brisket. Cover with plastic; chill overnight.
Mop: Mix first 6 ingredients plus reserved dry rub in heavy medium saucepan. Stir over low heat 5 minutes. Pour 1/2 cup mop into bowl; cover and chill for use in sauce. Cover and chill remaining mop.
Preheat oven to 375 degrees. Place brisket, fat side up, in a Dutch oven or other heavy baking pan large enough to hold the brisket. Roast brisket in pan for 3 to 4 hours, basting with mop every 20 minutes, until tender.
Transfer brisket to platter; let stand 15 minutes. (Can be made 1 day ahead. Wrap in foil; chill. Before continuing, rewarm brisket, still wrapped, in 350 degree oven about 45 minutes.)
Combine barbecue sauce and chili powder in heavy small saucepan. Add any accumulated juices from brisket and bring to boil, thinning sauce with some of reserved 1/2 cup mop, if desired. Thinly slice brisket across grain. Serve, passing sauce separately.