These tender waffles need time to set in the waffle iron before you lift the lid to check on them. Let patience rule, because peeking too soon will tear the waffle and make it difficult to remove. The sauce can be prepared ahead of time and stored, covered, in the refrigerator for up to 4 days. Bring to room temperature or heat before serving. If your waffle iron is not nonstick, brush it lightly with oil to prevent sticking.
To make Rhubarb Sauce: Combine rhubarb and sugar in a saucepan, and cook over medium-low heat until rhubarb is tender and translucent, about 15 minutes. Using a slotted spoon, transfer about 1 cup of rhubarb to a small bowl, and reserve for waffle batter. Increase heat to medium, and cook remaining rhubarb in syrup, stirring occasionally, until slightly thickened, for about 5 minutes.
To make Waffles: Whisk eggs in a mixing bowl until frothy. Whisk in milk and oil. Stir in reserved 1 cup cooked rhubarb. Sift flours, sugar, baking powder and salt into a mixing bowl. Gently stir dry ingredients into egg-milk mixture just until moistened.
Preheat a waffle iron to medium-high. Fill the iron about two-thirds full. Close, and cook waffles until they are nicely browned, for about 4 minutes. Repeat with remaining batter, coating waffle iron lightly with oil, if necessary, before cooking each batch. Serve waffles hot, topped with Rhubarb Sauce.