Apparently the people behind A Very Potter Musical are working on a Batman musical for the internets. This sounds like it could be fairly entertaining.
Fun fact: There was supposed to be another Batman musical in 1998 with music by Meatloaf lyricist Jim Steinman. What is available from the score is available here, and is insanely awful, and makes Spider-Man: Turn Off The Dark seem like Les Miserables in comparison. Totally worth a listen, you guys.