In a small saucepan over medium-high heat, combine filling ingredients and bring to a boil. Reduce heat to med-low and simmer until rhubarb is tender and broken down, about 8-12 minutes. Set aside to cool.
To make squares:
Preheat oven to 350ºF. Butter or spray a 8x8-inch baking pan.
In a large bowl, combine flour, oats, brown sugar and coconut. Add cold butter and work into the dry ingredients with your fingers until mixture looks like coarse meal the size of peas. Reserve 2 cups of crumble, set aside.
Press remaining oat mixture into bottom of prepared baking pan. Evenly spread rhubarb filling onto bottom crust. Top filling with reserved crumble. Bake 40-45 minutes until top is slightly brown. Let cool completely before cutting into squares.