Preheat oven to 350 degrees. Toast coconut on a rimmed baking sheet in oven, stirring often, until golden brown, about 10 minutes. Let cool completely.
Stir together flour, granulated sugar, coconut, nutmeg, baking soda, and salt in a medium bowl. Stir in water, 1 tablespoon at a time, until mixture forms a stiff dough (about 5 tablespoons water total). Form the dough into a rectangle. Wrap dough in plastic, and refrigerate until firm, about 15 minutes.
Preheat oven to 375 degrees. Roll out dough to 1/8 inch thick on a lightly floured piece of parchment paper. Transfer dough on parchment to a baking sheet. Refrigerate until firm, about 15 minutes.
Cut dough into 40 small rectangles (about 1 1/4-by-2 1/2 inches each) using a sharp knife. Transfer biscuits to parchment-lined baking sheets using a thin spatula. Lightly brush tops with water. Sprinkle with sanding sugar. Bake until biscuits just turn golden brown, about 11 minutes. Let cool on sheets on wire racks.
Biscuits can be stored in an airtight container, up to 1 day.