Heat gas or charcoal grill. Spray 25x18-inch sheet of heavy-duty foil with cooking spray. Without cutting all the way through, cut loaf of bread into 12 (3/4-inch) slices, cutting to within 1/4 inch of bottom.
Stir together butter and mustard. Spread every other slice of bread with slightly less than 2 teaspoons mustard mixture, creating 6 sandwiches. Fold each slice of cheese in half diagonally; tuck each into sandwich. Divide ham evenly among sandwiches, tucking in to fit. Place loaf on center of foil. Seal edges, making tight 1/2-inch fold; fold again, allowing space for heat circulation and expansion.
Place foil-wrapped loaf on grill over medium heat. Cover grill; cook 20 to 25 minutes or until cheese is melted and loaf is hot, turning loaf frequently. To serve, open packet carefully to allow steam to escape. Remove foil from loaf; pull apart sandwiches.