Hilarious in Hindsight: A Dracula musical was eventually produced and performed on Broadway. No puppets, unfortunately.
Old Shame: Jason Segel revealed in interviews that the Dracula puppet musical Peter is writing in the movie is, in fact, based on a real Dracula musical which he began writing in his youth, and the song he sings in the karaoke bar is a real song from that musical. He claims to have played a demo tape for Judd Apatow, whose only response was "You can never let anyone hear this tape." Listen for yourselves. The anecdote starts at 13:30.