Season wings with salt and 1 teaspoon pepper. Heat grill to medium high.
Place seasoned chicken wings and plum halves, cut side down, on grill. Reduce heat to medium and close grill cover. Grill plums for about 4 minutes on each side until soft but not falling apart. Remove plums to the bowl of a blender of food processor.
Increase heat on grill to medium high. Continue to cook wings for about 8-10 minutes. Turn and grill for 10-15 minutes. Wings should register 170º F when tested with an instant read thermometer. Remove to a serving platter.
While wings are grilling, to the plums add vinegar, hoisin sauce, ginger, soy, ¼ teaspoon pepper, and scallions. Process until smooth. Measure out ¼ cup plum coulis and reserve remaining sauce.
During the last 3-4 minutes of grilling, baste wings with ¼ cup plum coulis.
Drizzle wings with remaining coulis or serve coulis in a bowl on the side for dipping. Sprinkle wings with chopped cilantro and serve immediately.