Sounds like a good idea
ill try and get something out for you that might do the trick in the next couple of days.
Dont hold me to this, as Ive always had a hell of a time with SMF postbox, is never works with just an include.
For the other stuff, you can already do this if you set a board to "Local" and only allow a user to reply to their own topics.
If you dont see the "Local" switch in the "Permissions by Board" page, you will need to enable "advanced board permission" in the settings for permission section of your admin area