Preheat oven to 350. Coat a spring-form pan with baking spray and dust with flour.
In a medium-size bowl, sift together flour, baking powder, and salt.
In a large bowl, combine 1 cup of sugar, egg, milk, vanilla, and orange zest. Gradually add in the flour mixture, stirring until combined. Add the melted coconut oil and mix until thoroughly incorporated.
Arrange large chunks of rhubarb on the bottom of the pan. (Alternately, these can be mixed into the batter or arranged on top; your choice.) Spread the cake batter over the rhubarb. Sprinkle small rhubarb slices and flaked coconut over the top. Dust with remaining 2 Tbsp sugar.
Bake until a toothpick inserted in the center comes out with moist crumbs, 35-45 minutes. Let cool completely in the pan on a wire rack.