Preheat the oven to 400 degrees F.
Poke the potatoes, place on a parchment-lined baking sheet and rub all over with the olive oil and a generous sprinkling of salt and pepper. Bake until a paring knife easily pierces the flesh, about 1 hour. Let cool slightly.
Put the kielbasa in a large skillet with the veggie oil and then turn the heat to medium. Cook, stirring occasionally, until the fat starts to render, and the sausage is golden in places, about 5 minutes. Add the coleslaw mix and stir until just wilted, about 2 minutes. Transfer to a large bowl.
Cut the potatoes in half lengthwise and scoop out the flesh, leaving a 1/4-inch-thick wall of potato and skin so they hold their shape. Add the potato flesh to the bowl with the kielbasa mixture and stir in the butter, scallions, 1 cup of the mozzarella, 1/4 cup of the barbecue sauce and 1 teaspoon salt. Season with salt and pepper. Put the potato boats on the baking sheet and spoon the filling into them. Sprinkle the remaining cheese evenly over the tops. Bake until the cheese is melted and the tops are browned in spots, about 20 minutes. Brush with the remaining barbecue sauce, top with the reserved scallions and serve immediately.