Combine 3/4 cup sugar, flour, and salt in top of a double boiler; add milk, stirring well. Cook over boiling water, stirring constantly, 20 minutes or until thickened.
Beat egg yolks until thick and lemon colored. Gradually stir about one-fourth hot mixture into yolks; add to remaining hot mixture. Cook 15 minutes over boiling water, stirring frequently. .Remove from heat; stir in butter and bourbon. Cool.
Beat egg whites (at room temperature) until foamy; gradually add remaining 1/4 cup sugar, 1 tablespoon at a time, beating until soft peaks form. Fold egg whites into custard mixture; spoon filling into coconut piecrust. Sprinkle with toasted coconut. Chill until set.