Roll half of pastry to 1/8-inch thickness on a lightly floured surface; fit pastry into a 9-inch pie plate, leaving overhang. Set aside.
Combine 1 1/2 cups sugar, cornstarch, and nutmeg in a medium saucepan; stir mixture to remove lumps. Stir orange juice into sugar mixture. Cook over medium heat, stirring constantly, until mixture thickens. Remove from heat. Add rhubarb and butter, stirring until butter melts. Spoon rhubarb mixture into prepared pastry shell.
Roll remaining pastry to 1/8- inch thickness, and cut into fourteen 10- x 1/2-inch strips. Arrange strips in a lattice fashion over filling; press ends gently to seal strips to pastry overhang. Trim and flute. Brush with egg white, and sprinkle with remaining sugar.
Bake at 450° for 10 minutes. Reduce heat to 350°, and bake an additional 30 minutes or until crust is lightly browned. Cool before serving.