Combine sugar and water in a saucepan; bring to a boil. Remove from the heat; add butter and rhubarb. Cover and let stand 15 minutes. Drain, reserving liquid. Blend liquid with egg and vanilla. Combine bread cubes, rhubarb mixture, egg mixture and 3/4 cup coconut. Place in a greased 1 quart casserole. Sprinkle with the remaining coconut. Bake at 325 degres for 45 minutes or until set. Yield; 6 servings.