The thing here is that the chat DOM is handled by Ember, which means that manipulating it manually might give unexpected results.
Also you might have issues with your addon clashing with other addons, such as bttv.
An option would be if you find a way to manipulate the existing data which ember uses to render the chat. But that requires a lot of digging through the source.
I think a better idea for posting news would be if you just add a notice div somewhere, rather than hijacking the actual chat log.
But then again, you could probably just wing it and append a <li> to .chat-lines.