Shoutbox

Nope, that would be too risky and buggy... Messenger is actually very well protected against outside programs trying to send messages automatically (from the UI, not the protocol). And I find it personaly more useful to see the window opening, that way, you know you sent a message to your contact and you can expect an answer (or send an explanation such as "wops, I wrote than a month ago, just don't worry about it").