In a medium bowl, mix together 1 cup flour, confectioners' sugar, and butter until crumbs about the size of peas form or a soft dough forms. Press into the bottom of a 6x9inch baking dish. Bake this crust for 10 minutes in the preheated oven.
While the crust bakes, whisk together the remaining ingredients except the rhubarb and coconut in a large bowl. Fold in the rhubarb and stir to coat.
Remove the crust from the oven and allow it to cool slightly, then spread the rhubarb mixture over the crust. Sprinkle the coconut on top, if using.
Bake for 35minutes. Check to make sure the rhubarb is tender, then remove from oven and allow to cool. Cut into bars to serve.