Scoop out the pulp, leaving a 1/4-inch thick shell for the toppings. Cut the potatoes into quarter wedges, brush all over with butter and season with salt and pepper.
Place on grill and cook until crisp, about 10 minutes. During the last few minutes of grilling, remove potatoes and place on a cookie sheet. Top the potatoes with cheese and bacon and place back on top of grill.
While potatoes are grilling, combine the sour cream and horseradish in a small bowl and mix well.
Place potatoes on serving plates and top with the green onions, chopped tomatoes and the sour cream horseradish sauce. Sprinkle with chopped chives, for garnish.