Pour oil into a heavy medium pot to a depth of 2? and heat over medium heat until temperature registers 350º on a candy thermometer.
Meanwhile, combine corn flour, all-purpose flour, garlic powder, black pepper, cayenne, and salt to taste in a wide dish. Working in small batches, dredge oysters in flour mixture, shaking off excess, then deep-fry in the hot oil until golden brown, about 2 minutes. Transfer oysters to a wire rack to let drain briefly.
To assemble, spread mayonnaise on cut length of bread, then put half the pickles, oysters, tomatoes, and lettuce, in that order, on each bottom half. Place top halves of bread atop lettuce.