Heat the oven to 400°F and grease a 2-quart baking dish. Cook macaroni in a large pot of water with the tablespoon salt until 1 minute short of the package time, then drain.
While the pasta cooks, warm pork, barbecue sauce and water in a covered skillet over medium-low heat for 5 to 7 minutes until steaming; stir to prevent sticking.
Melt butter in a saucepan, whisk in flour for 1 minute and gradually add milk. Add mustard, sauce salt and pepper and simmer for 3 minutes; remove from the heat and stir in cheddar.
Fold the macaroni into the sauce and spread in the dish. Spoon the hot pork loosely over the surface, leaving some cheese-coated pasta visible.
Bake for 15 to 20 minutes until bubbling and 165°F in the center of the pork and pasta. Rest for 5 minutes before serving.