To prepare sherbet, combine sugar and 1 cup water in a small saucepan; bring to a boil over medium-high heat. Cook until sugar dissolves, stirring frequently. Transfer syrup to a large bowl; cool. Add buttermilk, yogurt, lemon rind, and juice, stirring with a whisk. Pour mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon sherbet into a freezer-safe container; cover and freeze 30 minutes or until firm.
Combine liqueur and orange rind in a small bowl; let stand 20 minutes. Serve with sherbet and berries. Garnish with mint.