Why stop at West Wing guest stars? Work with me for a second here. Just from the episodes I've watched in the past week, I've learned that Sam Seaborn spent two years as the recording secretary of Princeton's Gilbert and Sullivan Society, and heard him sing selections from Pirates of Penzance. Josh Lyman made a Brigadoon reference a few episodes back, and started singing "The Wells Fargo Wagon" in that fateful Oliver Platt episode. These guys are just aching for a song and dance number, you can tell.
Sam as Sky Masterson, Josh as Nathan Detroit. And if you really wanted to push it, Donna Moss as Adelaide, and Ainsley Hayes as Sarah Brown. President Bartlet, Big Jule, and scene.
But seriously. How fantastic would it have been if the West Wing had gone all Buffy/Scrubs/Even Stevens and filmed an entire musical episode?