In a small saucepan, combine vinegar, brown sugar, remaining lemon juice and black pepper. Boil until reduced by about 1/2.
Place peaches cut-side down on a hot lightly oiled grill. Cover and cook for about 2 minutes, or until there are grill marks. Turn over and baste the cut sides with the vinegar mixture. Cover the grill and cook for about 3 more minutes, or until softened.