1. Prepare charcoal fire for grilling, arranging coals around edge of firebox. Place foil drip pan under grilling area. Place ham, fat side down, in 3-quart casserole; add water. Cover tightly and microwave on high 5 minutes; turn ham over.
2. Cover tightly and microwave on medium (50%) 12 minutes. Mix remaining ingredients; reserve.
3. Insert barbecue meat thermometer in center of ham. Place ham, fat side up and 5 to 6 inches from medium coals, on grill over drip pan. Cover and grill 1 hour to 1 hour 15 minutes. Score top surface lightly, cutting uniform diamond pattern 1/4 inch deep and spoon plum glaze over ham occasionally during last 10 minutes of grilling, until thermometer registers 140°. Serve with any remaining plum glaze.