Hey,
I just thought of this right now. Why not have the Plus! Extention automatically resend messages when MSN alerts the user that the message has not successfully been sent to all recieptents. If the user is online, it will resend the message every 'x' ammount of seconds and if the user is now offline, it can automatically put it in the offline message window.
I just get annoyed when a contact is actually online and for whatever purpose it doesn't actually send to the other side.