Cooking brisket with the BBQ Guru will allow you to maintain temperature without a water pan. I cook briskets on my WSM with an ATC and routinely foil over my water pan and use it strictly as a heat defuser. That said, you will want to insure that the brisket does not dry out and there are several approaches for doing that.
1. Introduce water into the cooking system.
You would think that adding water into the WSM water pan would address this but in my opinion there is a big problem with that approach. The WSM water pan is directly under the brisket and will catch all the rendered fat. Fat floats on top of water and would inhibit moisture from evaporating from the water pan to humidify the smoke chamber. For that reason, I would not recommend that approach.
I would suggest that you get a disposable foil loaf pan, fill it with hot water, and set it next to the brisket on the WSM grate.
2. Spritz the brisket and wrap with foil or butcher paper at the stall.
Briskets do not lose much moisture until they hit the stall (between 160F and 170F) at that temperature the meat tightens up and squeezes water out. The evaporation of that water from the surface of the meat causes its internal temperature to stay constant (stall) until the water content of the meat is reduced enough to allow the temperature to rise. Too much water squeezed out equals a dry brisket flat.
You could try to spritz periodically after the stall for the next 8 or so hours until the brisket is done and avoid wrapping it. To me that seems like an awful lot of work.
If it were me, I would take Aaron Frankin's approach and once I hit the stall and had a nice bark formed, I would liberally spritz the brisket with water, apple juice, 50/50 water and Worcestershire sauce, broth, whatever your fancy and then wrap with butcher paper until done. The butcher paper is semi-permeable and keeps moisture in but allows for a nice bark to continue to form and does not steam the brisket in its own juices like foil would do.
I would suggest that you take a look at the
BBQ with Franklin videos. Texas monthly ranked him number one in Texas brisket. There are three videos on how he does his brisket. I think you can adapt his methods to your WSM and BBQ Guru.
Good luck,
-- Mache