PLACE GRANULATED SUGAR AND BUTTER IN A LARGE BOWL; BEAT WITH A MIXER AT MEDIUM SPEED UNTIL WELL BLENDED. ADD EGGS, 1 AT A TIME, BEATING WELL AFTER EACH ADDITION. ADD BANANA, YOGURT, RUM, AND VANILLA; BEAT UNTIL BLENDED. ADD FLOUR MIXTURE; BEAT AT LOW SPEED JUST UNTIL MOIST. STIR IN 1/2 CUP COCONUT. SPOON BATTER INTO A 9 X 5-INCH LOAF PAN COATED WITH COOKING SPRAY; SPRINKLE WITH 1 TABLESPOON COCONUT. BAKE AT 350° FOR 1 HOUR OR UNTIL A WOODEN PICK INSERTED IN CENTER COMES OUT CLEAN. COOL IN PAN 10 MINUTES ON A WIRE RACK; REMOVE FROM PAN. COMBINE POWDERED SUGAR AND JUICE, STIRRING WITH A WHISK; DRIZZLE OVER WARM BREAD. COOL COMPLETELY ON WIRE RACK.