In processor, process plums, onion, garlic and ginger until finely chopped. In saucepan, combine plum mixture, sugar, vinegar, cinnamon, salt and pepper flakes. Simmer, uncovered, stirring occasionally, 30 minutes, until thickened (about 2 cups). Remove to bowl; let cool.
Place 1 cup plum ketchup in large plastic food-storage bag. Refrigerate remainder. Add soy and pork to bag. Refrigerate 1 hour or overnight, turning occasionally.
Heat broiler. Remove pork from marinade; place on broiler-pan rack. Broil 4 inches from heat 18 to 20 minutes, turning twice, basting with marinade, until 160 degrees F. Let rest 10 minutes. Slice; serve warm or chilled, with ketchup. Makes 10 servings.