Combine rhubarb, 1/4 cup water, and salt in a food processor or blender; blend until rhubarb is finely chopped.
Place a cheesecloth onto a jug and pour the blended rhubarb onto it. Tie the cheesecloth and squeeze all the juice out of the rhubarb. Discard the pulp.
Pour maple syrup into the jug and mix well until all the maple syrup has dissolved.
Squeeze limes into the jug and mix well. Add ice cold water and mix everything together with a large wooden spoon. Taste and adjust sweetness if necessary. Refrigerate rhubarb limeade in the refrigerator until ready to use.
To make slushies: For each serving, place 3/4 cup of rhubarb limeade into the blender with about 1 cup ice; blend until desired consistency.