Tear off 4 sheets of nonstick foil. Mound 1 1/2 tablespoons light brown sugar and 1 tablespoon unsalted butter on each sheet. Top each mound with a 1/2-inch-thick pineapple ring and put a maraschino cherry in the middle, then top each with an upside-down shortcake shell. Fold in the sides of the foil and seal to form a packet. Grill over medium-high heat, 12 minutes. To serve, flip the cakes pineapple-side up.