To make the chipotle mayonnaise: Whisk together mayonnaise, chipotle, and lemon juice and chill mixture, its surface covered with plastic wrap.
To make the oysters: Heat oil in a deep heavy pot (preferably a cast iron Dutch oven) over high heat until it registers 375 degrees F on deep-fat thermometer, about 12 minutes.
While oil is heating, whisk together egg, milk, and 1 teaspoon salt in a bowl. Shake cornmeal, remaining 1 1/2 teaspoons salt, and pepper in a plastic or paper bag until combined well.
Working in batches, add oysters to egg mixture, then lift out, letting excess drip off, and transfer to cornmeal in bag, shaking to coat well.
Carefully transfer to oil, knocking off excess coating, and fry, turning occasionally, until golden and just cooked through, 1 to 2 minutes. Transfer with a slotted spoon to paper towels to drain. Coat and fry remaining oysters in same manner, returning oil to 375 degrees F for each batch.
To assemble the sandwich: Halve loaf crosswise and horizontally, cutting all the way through, and spread 1 cut side of each piece with mayonnaise. Sandwich oysters and lettuce between bread, pressing gently.