In a bowl, whisk the mayonnaise with the hoisin sauce, pickle relish and lemon juice.
In a large resealable plastic bag, combine the flour, garlic powder, onion powder, salt and white pepper. Seal the bag and shake. Add the fish and shake to coat.
Put the eggs and panko in separate shallow bowls. Dip the fish in the egg and then in the panko. Transfer the fish to a wax paper–lined platter.
In a large, deep skillet, heat the oil to 350°. Add half of the fish and fry over moderate heat, turning once, until golden, about 3 minutes. Drain the fish on a paper towel–lined platter. Repeat with the remaining fish.
Serve the fish in the tortillas, passing the hoisin mayonnaise, cabbage, lettuce, cilantro and scallions for serving.