Prepare barbecue (medium-high heat). Whisk first 3 ingredients in large bowl to blend. Add peach halves; toss to coat well. Place peaches, cut side down, on grill. Grill until slightly charred, about 1 minute. Using tongs, turn peaches over. Divide chopped white chocolate among peach cavities and drizzle remaining butter mixture from bowl over chocolate. Grill until chocolate just begins to melt and peaches are charred, about 2 minutes. Divide peach halves among bowls. Sprinkle with pistachios and serve.
