1. To prepare rémoulade slaw, combine first 9 ingredients in a medium bowl, stirring with a whisk. Add coleslaw; toss to coat. Cover and chill.
2. To prepare po'boys, combine cornstarch and next 4 ingredients (through egg white) in a medium bowl; whisk until blended. Add shrimp; toss well. Marinate in refrigerator 30 minutes, stirring once.
3. Hollow out top and bottom halves of bread, leaving a 1/4-inch-thick shell. Place torn bread in a food processor; process until very fine crumbs form. Set aside 1/2 cup breadcrumbs; reserve remaining breadcrumbs for another use. Combine 1/2 cup breadcrumbs, cornmeal, and black pepper in a large zip-top plastic bag; seal and shake to combine.
4. Remove shrimp from bowl; discard marinade. Add shrimp to breadcrumb mixture. Seal and shake to coat.
5. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add shrimp; cook 3 minutes on each side or until done.
6. Arrange 1/2 cup slaw on each bottom half of bread. Top with one quarter of shrimp and 2 tomato slices. Cover with top half of bread.