Heat oven to 350°F. Sprinkle 1 cup coconut on cookie sheet. Bake at 350°F. for 5 to 7 minutes or until toasted.
Meanwhile, grease and flour 13x9-inch pan. In large bowl, combine cake mix, water, 1/2 cup pineapple juice, oil, 1/4 cup rum and egg whites; beat at low speed until moistened. Beat 2 minutes at high speed.
Stir in 1/2 cup of the toasted coconut; reserve remaining 1/2 cup for frosting. Pour into greased and floured pan.
Bake at 350°F. for 25 to 35 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes.
In small saucepan, combine 1/2 cup pineapple juice and sugar; bring to a boil. With long-tined fork, prick cake at 1/2-inch intervals. Pour hot pineapple mixture over cake. Cool 1 hour or until completely cooled.
In small bowl, combine frosting and 1 tablespoon rum; blend well. Frost cake. Sprinkle with reserved 1/2 cup coconut. Refrigerate until serving time.