Preheat oven to 375 degrees F.
Brush six 5-inch diameter, 1-inch high ovenproof dishes with 1 Tbsp. melted butter; place dishes on rimmed baking sheet. Cook diced plums, 4 Tbsp. sugar, and 2 Tbsp. rum in medium skillet over medium-high heat until juices bubble thickly, stirring often, about five minutes. Cover; cook until plums are just tender, stirring occasionally, three minutes.
Using slotted spoon, divide diced plums among prepared dishes. Add orange juice concentrate to juices in skillet; stir to blend. Pour into measuring cup. Add remaining 2 Tbsp. rum, then cream to measure 3/4 cup total.
Whisk 1/2 cup plus two Tbsp. sugar and three Tbsp. flour in large bowl. Whisk in cream mixture, eggs, and remaining three Tbsp. melted butter. Spoon batter over plums in dishes. Top with plum slices; sprinkle with remaining one Tbsp. sugar.
Bake clafoutis until puffed and crusty and center is just set, approximately 30 minutes. Cool 15 minutes and serve warm with vanilla and/or ginger ice cream and dust with powdered sugar.