I have only just recently began looking at Question2answer. However I think I might be able to help you.
The logo is located in the qa-include/qa-theme-base.php
Now, search for $this->content['logo'], after you add next to it:
$this->output('YOUR CODE BANNER HERE');
And it should work. {Remember to backup your file before you try this}
I don't know if this is the correct way, I've never worked with such a difficult system for themes, but this worked for me.