Actually, I did find a sugar-free rhubarb pie recipe:
Sugar Free Rhubarb Pie
The thought of sugar free rhubarb recipes makes your lips pucker. But, sugar free recipes that use rhubarb are easier than you think. For example, take rhubarb cake recipes or rhubarb muffin recipes. When directions call for sugar, replace it with fruit juice concentrates or mild tasting fruit, such as pears or Delicious apples. Here's a sugar free rhubarb pie recipe that will make you a believer.
4 cups diced, raw rhubarb
2 cups diced peeled sweet apples, such as golden delicious
1/3 cup apple juice concentrate
1/4 cup unbleached flour
1 tsp. ground cinnamon 1 tablespoon grated orange rind
Line a pie pan with pie dough. Stir the rhubarb and diced apples together and arrange them on top of the pie shell. Combine the remaining ingredients and sprinkle them over the fruit. Dot with 2 tablespoons butter (optional). Cover the pie with well-pricked pie dough or with a lattice. Bake the pie in a 450-degree oven for 10 minutes. Reduce the heat to 350 degrees and bake for 35 minutes or until golden brown. Enjoy!
This is from http://www.plantea.com/rhubarb.htm