Preheat oven to 350 F. Butter a 9-inch springform pan. Line bottom with parchment paper. Butter the parchment.
In a bowl combine rhubarb, 1/2 cup sugar and orange zest. Mix until rhubarb is evenly coated. Set aside.
In a medium sized bowl combine remaining sugar, both flours and baking soda.
In a separate medium sized bowl add coconut milk, coconut oil, vinegar and vanilla extract. Let stand one minute until milk curdles. Fold wet ingredients into dry ingredients. Do not over mix.
Fold in shredded coconut.
Pour rhubarb into the bottom of the prepared springform pan and spread in one even layer. Top rhubarb off with coconut cake batter.
Bake cake for 1 hour, or until a toothpick inserted into the center comes out clean.
Allow cake to stand until cool. Release the sides of the springform pan and invert onto a cake plate.