>they're not legally entitled to much privacy in these places.

While I think this is a really cool project, I also agree with the privacy issues. CA is a two party consent state, and recording a conversation (which this is likely to do) like this is likely illegal. While a person might not have a expectation of privacy about someone just hearing the conversation, they are protected by law if they are recorded without their knowledge.

NB: I am not a lawyer, and the above could very well be wrong.

Edit: As I was informed below, I was wrong on the legal points.