I had an idea, why not have an option in the CP where you have a choice of sending mass PM's to users in your board.
You can already do this by emailing everyone, but I would find this much more usefull!
Basically format it EXACTLY the way the email works, but where there is a Subject line it would be the Topic line.
You would be able to choose the users the same way as in the email system as well! Just a thaught! Tell me what you think and what suggestions you have!