1. Fold a 20-inch sheet of wax paper lengthwise into thirds. Wrap around the outside of a 1-quart souffle dish and attach with tape to form a very tight seal; place in freezer to chill.
2. Puree cubed cantaloupe in blender, pulsing until smooth. Transfer to medium metal bowl. Stir in lemon juice.
3. Sprinkle gelatin over 1/4 cup cold water in small bowl; let stand 5 minutes to soften. Microwave on High 30 seconds; stir until gelatin is completely dissolved. Stir into cantaloupe puree, then stir in corn syrup. Place medium bowl in larger bowl filled with ice cubes and water. Let stand 20 to 25 minutes, stirring occasionally with a rubber spatula until mixture has thickened.
4. Meanwhile, combine sugar and 1/2 cup water in small saucepan; add vanilla bean, if using. Cover and bring to a boil over medium heat; boil 5 minutes. Uncover; remove vanilla bean with slotted spoon and transfer to a plate. Continue cooking syrup 8 to 10 minutes more or until syrup reaches thread stage (235 degree F on a candy thermometer).
5. Beat egg whites in large bowl of a standing mixer at medium speed to soft peaks. With mixer on low speed, add hot syrup into whites in a slow, steady stream, pouring along edge of bowl. Increase mixer speed to high; beat until meringue is completely cool, 5 minutes.
6. Scrape vanilla seeds from bean with small knife. Beat seeds or vanilla extract into cooled meringue.
7. Fold meringue into melon mixture with a spatula. Gently cut down through the middle of the mixture to the bottom of the bowl, then up the side and over, slightly covering the meringue with some melon mixture. Turn the bowl slightly and repeat. Pour into prepared souffle dish. Freeze mousse overnight. (Can be made ahead. Cover and freeze up to 1 week. Let soften at room temperature 5 minutes before serving.)
8. With assorted-size melon ballers, make scoops of cantaloupe, honeydew and watermelon. Garnish mousse with melon balls, blueberries and fresh mint sprigs. Makes 6 servings.