That does sound like a good idea; in the mean-time, though, there is a way you could do something like that ... just manually.
You'd have to have the "sticky threads' hack" installed; create your post and put announcement in the topic heading; click the close box; send it to the board.
When the announcement runs its course, you'd just "unstick it."
A lot of little work, so the idea of an announcement from the CP does sound like a sweet idea.