Grate enough zest, then squeeze enough juice from 1 orange to measure 1 Tbsp. plus 1-1/2 tsp. zest and 1/4 cup juice. Mix zest and juice with barbecue sauce, sugar, vinegar, onions, garlic and pepper until blended. Pour over chops in shallow dish; turn to evenly coat chops. Refrigerate 4 hours to marinate.