Unfortunately, I have to say this: impossible with a skin.
Seriously, there's
no way you can achieve that using UIFILEs. You'd have to go code it, hook into Messenger and try to get the messages appear like that, however don't count on any willing to spend his/her time on that. But I can assure you: UIFILEs can't do that.
It's an awesome idea, but here I have to tell you that the sky isn't always the limit (as Patchou claimed in his news post about skinning). Sorry!