I'm going to go ahead and disagree with you on this... but for other reasons. While yes, a post has been sent to the server and it would be possible to capture, the security policy would be violated. Hence, if it doesn't know who you are, then it has to keep the message around as a guest... but then how does it know which guest is which? Ahh by session. But you don't have a session, so what happens then? Well, we could track it by connecting IP address, but then what if for example you are an AOL user behind a proxy? You could have many members hitting the site at the same time... what then?
See, the more ratholes you go down, the more it becomes clear that creating a system such as this would be improbable and probably dangerous. But by all means, I see that you are passionate about this problem. I suggest going to the Simple Machines website, read up on the underpinnings of the code, and create a mod that can accomplish what you are trying to do. If you do, I'd be happy to review it and put it in the forum.