While oil heats, pour buttermilk into a shallow dish. Place crabs in buttermilk; turn to coat and let stand 5 minutes. On another plate, mix flour, cornmeal, 1/2 teaspoon salt and 1/2 teaspoon pepper. Season crabs with 1/4 teaspoon salt and 1/4 teaspoon pepper, then remove crabs from buttermilk one at a time and coat with flour mixture. Place crabs on a baking sheet.
Carefully add crabs, four at a time, to hot oil. Cook until golden, about 1 1/2 minutes per side. With a slotted spoon, transfer crabs to paper towels to drain.
Spread mayonnaise on cut sides of bread. Top each bottom half with two lettuce leaves, two pickle slices and tomato slices. Sprinkle tomatoes with the remaining 1/4 teaspoon salt. Top with crabs. Add top halves of bread. Serve with hot sauce.