Spread blueberries on the bottom of a 11x7 greased baking dish. Squeeze the juice from the lemon over them. In a medium bowl, stir together the margarine and sugar until smooth. Stir in flour and baking powder alternately with rice milk until smooth. Batter will be thin. Spoon over the berries, and spread evenly. To make topping, in a small bowl, stir together sugar, salt, cornstarch, and cinnamon. Sprinkle over the batter, then pour boiling water over the entire dish. Bake at 350 for 45 minutes or until golden brown.