In a 2- to 3-quart pan over low heat, combine butter, egg yolks, sugar, lemon peel, and lemon juice. Stir constantly until mixture thickly coats the back of a wooden spoon, 10 to 12 minutes (do not boil). Pour into a small bowl, cover, and chill until cold, at least 2 hours, or nest pan in a large bowl of ice water and stir mixture until cold, about 10 minutes.
In a heatproof 1-cup glass measure, sprinkle gelatin over 2 tablespoons cold water. Let stand until soft, 2 to 3 minutes. Put 1 inch of water in a small pan; bring to a boil over high heat, then remove from heat. Set measure in water in pan and stir until gelatin is dissolved, 2 to 3 minutes. Let cool.
In a large bowl, beat cream until slightly thickened. Beating constantly, pour in gelatin; continue beating until soft peaks form.
Stir about 1/2 cup whipped cream into cold lemon mixture until well blended, then scrape lemon mixture into remaining whipped cream and fold gently until incorporated. Spoon into pie crust and spread level. Cover with plastic wrap and chill until firm, at least 1 hour, or up to 1 day.
Pour rhubarb compote into a fine strainer set over a bowl. Let drain until dripping stops, then gently spoon rhubarb pieces over lemon cream. Cut pie into wedges.