1. Place first 6 ingredients, 2 tablespoons salt, and 2 tablespoons syrup in a pan. Bring to a boil; dissolve salt. Cool. Seal pork and brine in a zip-top plastic bag. Chill 8 hours; drain.
2. Preheat grill to medium-high heat.
3. Sprinkle pork with 1/4 teaspoon salt and 1/4 teaspoon ground pepper. Grill pork on a rack coated with cooking spray 3 minutes on each side or until done. Combine 1 1/2 teaspoons syrup and butter; brush onto fruit. Sprinkle with 1/4 teaspoon salt and 1/4 teaspoon ground pepper. Grill fruit, cut sides down, on rack coated with cooking spray for 3 minutes. Cut each peach and plum half in half again; serve with pork. Top with onions, if desired.