Cuz, I agree with you...flat up. I was watching a video you sent me where Aaron Franklin smoked a brisket in an offset smoker, but I think his thinking applies here. His was horizontal on a rack, but he stated that he wanted the thickest part of the brisket closest to the heat source where it would sorta protect the thin flat from overcooking and drying out. If you think that the weight of the brisket might break free from the hook after it gets tender, leave the grate in place underneath for it to fall on.