Combine the first 4 ingredients in a medium saucepan, and stir well. Bring to a boil over medium-high heat; reduce heat, and simmer, uncovered, 4 minutes or until rhubarb is tender.
Place rhubarb mixture in food processor, and process until smooth; chill. Layer with the frozen yogurt in parfait glasses. Garnish with mint and crystallized ginger, if desired.