Arrange the potatoes in 13 by 9 by 2-inch glass baking dish. Combine the cane syrup, butter, and salt in a small saucepan over medium heat. Bring to a boil, then remove the pan from the heat. Stir in the orange zest and liqueur. Pour the sugar mixture over the potatoes, and toss the potatoes to coat. Cover the dish tightly with foil. Bake for 50 minutes, and then remove the dish from the oven, remove the foil, and bake until the potatoes are tender and the syrup has thickened slightly, basting occasionally, about 20 minutes.
Combine the butter, brown sugar, flour, cinnamon, salt, and pecans in the bowl of a food processor fitted with a metal blade, and pulse the mixture until it resembles coarse pebbles. Transfer the mixture to a bowl, then add the marshmallows and toss to combine.
Raise the oven temperature to 425 degrees F. Remove the baking dish from the oven and cover the potatoes with the marshmallow streusel. Return to the oven, and bake until the topping is bubbly and browned, about 10 minutes.