In case it's not obvious....
...I quit smoking a couple of days ago, so my thoughts are not necessarily properly filter, sequential or coherent. :-)
The one thing you have going for you in a compressed scheme is that most users won't have a lot of individual unread messages (or killed messages) for an extended period of time. In fact, everytime you want to mark (or kill) a message, there has to be a roundtrip to the server. So, unless something really bugs you, or you really really want to come back to a message later, you'll probably not hassle with micro-managing the stuff. This should mean that you'd be able to compress down to a much more manageable scal of data, and as long as it doesn't overly bog down the normal queries it would be feasible.
One other note, if you're gonna kill an individual, you may want to kill them on an individual forum basis - perhaps I don't like what a person has to say about Politics, but they happen to be the best person to talk to about Linux.
As for scoring, I'd think you'd need a much larger population of both users and messages to make it worthwhile.
