Place potatoes on grill and close lid. Grill for 1 hour. Then remove from heat and let cool just until you can touch them comfortably, about 10 minutes. Cut them open and remove the cooked potato flesh.
In a mixing bowl, whip the potato flesh, sour cream, milk, butter, salt, pepper, 2/3 cup cheese, and 4 of the chopped green onions. Then replace in the potato skins, sprinkle with cheese, green onions, and bacon, and return to the grill.