Shoutbox

Sending WM_CHAR messages? Sounds like you're writitng your own SendKeys. Mind you, it shouldn't be sending the message to your contact, even if you send a newline.
If you think it may be useful, look up Active Accessibility. You can use AccessibleObjectFromWindow on the HWND returned by the interface passed in the PLUGIN_PARAM (typecasted to IMessengerConversationWnd), then you can use the various other functions like AccessibleChildren to get the Input box, and set it's value.
It won't be simple, and will require testing and debugging. However, if you get it working, it'll be much better than SendKeys or WM_CHAR