I generally don't bother rubbing the fat cap, since it will end up rendering off or getting trimmed. That being said, it usually ends up acquiring a fair amount of rub anyway.
I wouldn't bother with trying to sear a brisket. It's a whole different animal from prime rib (although it is from the same animal). Just build a coal bed on one side of the kettle, put the brisket on the opposite side, put on the lid and smoke away! And don't peek! Keep us posted, and post pics when you're done!
Weber Smokey Joe
18" Weber Kettle
22" Weber clone kettle
Curb-foundling char-broil gasser
WSM: Assembled! First cook: 8/25/06
Fanatical devotion to the Q