Combine the sugar and rum in a microwave-safe bowl. Microwave at high 1 1/2 minutes or until sugar dissolves. Brush rum mixture evenly over pineapple wedges.
Heat butter in a grill pan over medium-high heat. Add pineapple; grill 3 minutes on each side or until grill marks form and pineapple is thoroughly heated. Sprinkle with coconut. Garnish with ice cream, if desired.