Whisk flour, cornmeal, garlic powder, onion powder, and cayenne in a medium bowl; season with salt and pepper. Working in batches, toss oysters in flour mixture; transfer to a plate.
Working in batches, cook bacon in a large skillet, preferably cast iron, over medium heat until crisp, 10–12 minutes per batch; drain on paper towels.
Pour oil into skillet with bacon fat to a depth of 1" and heat over medium-high heat until oil bubbles immediately when a pinch of cornmeal mixture is added. Working in batches, fry oysters, turning occasionally, until golden brown and crisp, about 4 minutes. Drain on paper towels; season with salt and pepper.
Build sandwiches with brioche, mayonnaise, lettuce, tomatoes, bacon, and fried oysters.
