In a small saucepan over medium high heat, bring the vinegar to a boil and cook until it is reduced by half, 20-25 minutes. Stir in the molasses and black pepper. Remove from heat.
Place the peaches on the fire (grill, whatever), cut side down and grill until they are nicely brown (about 2 minutes). Flip them over, brush the cut side with the balsamic glaze, and continue to cook for another 2 minutes. Remove the peaches from the grill, give them another coat of glaze, and serve whole or sliced.
The recipe also says that the glaze "keeps forever, and it is very tasty when brushed on any kind of grilled meat during the last minute or two of cooking."