Jeez, briskets are tough enough in a WSM!
I'd probably recommend against it, but if you are determined, I'd do the indirect route with a trivit and aluminum foil. You need to run it at a lower temp to help break it down. For future reference, the offcial Aussie Weber trivit is approx 40cm x 30cm x 7.5cm, which roughly converts to 15-3/4" x 11-3/4" x 1/2". If you match that up to the grates, it's pretty close to the outside edges of the solid portion of the grates over the burners. Just cover that square with a sheet of HD aluminum, cut a few slits in the foil following the grates, then set down the trivit. I would probably some wood chips in some foil and set to the side of the grate for a little smoke. Then run the brisket straight on the trivit for 2 hours at around 275, and then either wrap it in butchers paper and put it back on the trivit, or put it in an aluminum pan, cover it with foil, and put the whole thing on the trivit. The 2 hours will get some smoke, and after that, it's all about saving the moisture and tednerizing as you finish.
That's what I would do at least, IF I had no choice but to use a Q320 for this cook. Again, I'd use the smoker for sure. Good luck.