Prep the oysters: Mix the cornmeal, flour, salt, pepper and cayenne together in shallow container. Scoop up about 8 of the oysters using a slotted spoon. Allow the excess fluid to derain off briefly. Scatter the drained oysters across the cornmeal mixture. Shake the container to coat evenly. Transfer them to a wire rack set over a rimmed baking sheet. Repeat with the rest of the oysters.
Heat oil in a large Dutch oven, with a candy thermometer attached to the side over high heat until it reaches 375 degrees. Slip half of the oysters carefully into the oil. Cook, stirring and prodding them to keep them from sticking together, until they are golden. It's about 2 minutes. Transfer them to a plate lined with paper towels.
Return the oil to 375 degrees and repeat with remaining oysters.
To make the sandwich: Hollow out the middle of the bagette pieces. Spread mayo liberally inside each hollow. Sprinkle with lemon juice. Salt and pepper it if desired. Spread the pickles and fried oysters into the bottom crusts. Top each sanddwich with some tomato slices and lettuce leaves, ending with the top bread half (-ves).
If you want to make shrimp sandwiches, substitute 3/4 lb. medium shrimp, peeled and deveined. Toss them with 1 large egg (slightly beaten). Use that instead of the oysters in their liquor.