Have butcher loosen back bone of meat. Insert tip of knife into meat at even intervals and push garlic halves into meat as you remove knife. For marinade combine wine, soy sauce, onion, vinegar, brown sugar, dry mustard, and ginger root. Place meat in a plastic bag; set in a baking dish. Pour marinade over meat; close bag. Marinate in refrigerator 10 hours or overnight, turning often.
Remove roast, reserving marinade. Pat dry with paper towels. Place rib side down in shallow roaster; insert meat thermometer. Roast uncovered at 325 degrees for 2 1/2 to 3 hours, until thermometer registers 170 degrees. Brush frequently with marinade during the last 30 minutes.
Combine 1/2 cup marinade with 1/4 cup water. Bring to boiling, reduce heat and keep warm. Arrange roast and luau sweet potatoes on platter. Serve with rice and heated marinade. Makes 6-8 servings.
