Heat oil in medium saucepan over medium heat. Press garlic through garlic press into pan. Stir 30 seconds. Mix in ketchup, 3/4 cup broth, 3 tablespoons soy sauce, brown sugar, molasses, and vinegar. Bring sauce to simmer. Reduce heat to medium-low; simmer until reduced to 3 cups, stirring occasionally, about 15 minutes. Transfer to bowl. Can be made 1 week ahead. Cover and chill.
Sprinkle each side of each rib rack with salt, pepper, and 1 tablespoons chili powder. Arrange 2 racks on each of 2 large rimmed baking sheets. Can be made 1 day ahead; cover and chill.
Position 1 oven rack in top third of oven and 1 rack in bottom third; preheat to 450 degrees. Pour 1/2 cup of remaining broth and 2 tablespoons of remaining soy sauce around ribs on each sheet; cover each with foil. Bake ribs 30 minutes; reverse sheets. Bake ribs until tender, about 45 minutes longer. Uncover; cool slightly. Cut each rib rack between bones into individual ribs. Brush ribs generously with sauce. Can be made 2 hours ahead. Let stand at room temperature.
Prepare barbecue (medium-high heat). Grill ribs until heated through and slightly charred, turning occasionally and brushing with more sauce, about 10 minutes. Pile ribs on platter.