Prepare a grill for medium-high indirect heat: For gas grills (with 3 or more burners), turn all the burners to medium-high heat; after about 15 minutes, turn off one of the middle burners and keep the remaining burners at medium high. For charcoal grills, bank one chimney starter-full of lit and ashed-over charcoal briquettes to one side of the grill. Set up a drip pan on the other side to avoid flare-ups. (Be sure to consult the grill manufacturer's guide for best results.)
Toss the Brussels sprouts with the butter, oil, 2 teaspoons salt and several grinds of black pepper in a medium bowl. Layer the bacon in a 24-by-12-inch grill basket in a single layer and top with the Brussels sprouts. Close the grill basket and place it on the grill over indirect heat Brussels sprouts-side down. Cover the grill and cook until the Brussels sprouts are tender and dark golden, 15 to 20 minutes. Open the grill, flip the basket so the bacon is down and move the basket to direct heat. Cook until the bacon is crisp, 2 to 3 minutes. If you want your Brussels sprouts darker, flip again and cook for another 1 to 2 minutes.
Carefully remove the Brussels sprouts to a serving platter, crumble the bacon over the top and sprinkle with the lemon zest. Sprinkle with the red pepper flakes and Parmesan, if using.