Cut the pork tenderloin into 1-1/2-inch-thick slices. Wrap one slice of bacon around each piece of tenderloin. Thread tenderloin onto metal skewers. Thread mushrooms onto a second set of skewers. Thread corn and onion wedges onto a third set of skewers. Pit 8 of the plums; thread ontio a fourth set of skewers. Brush skewers with cooking oil.
Grill pork and corn skewers on grill rack of an uncovered grill directly over medium heat for 16 to 18 minutes or until an instant-read thermometer inserted into thickest portion of meat reads 160 degree F. Halfway through grilling time, turn skewers and add mushroom skewers. During the last 5 minutes of grilling, add plum skewers and asparagus spears. Turn asparagus and plums once and brush everything with half of the maple syrup.
Meanwhile, for sauce, quarter and pit remaining plums. Heat butter in a medium saucepan. Cook and stir shallot in hot butter for 1 minute. Add vinegar, quartered plums, orange peel, cayenne pepper, and black pepper. Add orange juice. Bring mixture to boiling; reduce heat. Boil gently, uncovered, for 8 to 10 minutes or until thickened. Add remaining 1/4 cup maple syrup and heat through.
Place asparagus spears on a large platter. Arrange pork, plum, and vegetable kabobs on top. Serve immediately with warm sauce. Garnish with sprigs of fresh rosemary, if desired. Maks 4 to 6 servings.