Preheat oven to 350 degrees.
Place sweet potatoes in a large saucepan, and cover with water. Bring to a boil, then reduce heat and cook on medium-low until fork-tender, about 15 minutes. Drain well, and place in a large mixing bowl. Using an electric mixer, beat sweet potatoes until smooth. Add maple syrup, coconut milk, cinnamon, ginger, vanilla, and salt, and beat until well combined. Add pineapple and mix well.
Spread sweet potato mixture in a lightly oiled pie dish or square pan (or divide among ovenproof ramekins for an extra elegant presentation). Sprinkle coconut and walnuts on top of sweet potato casserole. Bake in oven for 25-30 minutes until warmed through. Remove from oven and set aside until ready to top with meringue.
Meanwhile, prepare meringue topping. Beat egg whites and salt in a medium bowl until soft peaks form. Slowly add sugar in a steady stream, while beating, until stiff peaks form.
Spoon or pipe meringue on top of sweet potato casserole.
Place sweet potato casserole on oven rack in top third of oven. Broil about 30 seconds, leaving the oven door partially open and watching carefully to prevent burning. Meringue burns quickly!