In a bowl, mix 1 3/4 cups flour and 1/4 cup brown sugar. With your fingers or a pastry blender, rub or cut in butter until mixture forms coarse crumbs. Spread mixture evenly over the bottom of a 10- by 15-inch baking pan; firmly press into a smooth layer. Bake in a 350° oven until slightly dry to the touch, about 10 minutes. Let cool for 15 minutes.
Meanwhile, in a bowl, with a mixer on medium speed, beat eggs lightly. Beat in salt, baking powder, remaining 1 cup brown sugar, and remaining 1/4 cup flour. Stir in walnuts, coconut, and lemon juice. Spread over baked crust and return to oven; bake until lightly browned, 20 to 25 minutes. Remove from oven; immediately brush with lemon glaze.
Let cool completely on a rack, then cut into 60 bars.
Lemon glaze. In a bowl, mix 1 cup powdered sugar, 1 1/2 teaspoons grated lemon peel, 2 tablespoons lemon juice, and 1 teaspoon warm water until smooth.