It's called the "shoutbox"
...and I just enabled it. I haven't spotted a kitchen sink yet, but damn, that software does seem to have everything.
Their "shoutbox" seems to be what you're describing: Anybody can post a persistent message there (I put it in the right column). Now you should post one, and let's see how it handles it; I'm guessing it displays only the last message, and the link "Read more" suggests it archives previous messages.