According to Commentary! The Musical, the woman seated next to Groupie #2 (wearing sunglasses, a dark wig and scarf) during the unveiling of the Captain Hammer statue is Sarah Michelle Gellar. After the release of the DVD, Joss Whedon set the record straight, pointing out the actress was, in fact, NOT Gellar.
Notice that Billy never puts on the goggles that he has on his head until the end of act 3 when he is accepted into the ELE and officially becomes Dr Horrible. Until that point they are merely a prop for a wannabe. The goggles would seem to signify that 'Dr Horrible is here' as the song lyrics would attest. Billy is no longer the dominant personality and Dr Horrible has taken over.
The song 'A Man's Gotta Do' is prophetic to events that occur later in the show. Captain Hammer sings twice to Penny during the song that 'The only doom that's looming is you loving me to death'. And of course it is precisely Penny's love for Hammer which leads to Dr Horrible's rash actions and her eventual doom.