This would be very useful to coordinate the effort while pushing the proposal forward. I don't think there is a way to do this at the moment because, as far as I know, you can't even see all the followers, but I can be wrong.
Yes, you can create a chat room. All of your followers (and everyone on Area51) can participate in the chat room. This can be a useful tool to coordinate plans. It fits well in the Stack Exchange communication model, as only those who are interested participate and are notified.
Chat has been used on several proposal with mixed results.
No, we do not provide the ability for proposal owners (or participants) to contact other followers by email. This is by design. We've always been ultra-cautious about when and how we contact users via email across all of our services; this has become ingrained in our culture. I know it is not always expedient or ideal, but communication and announcements have to be done through the proposal interface on the site.