I didn't know if its possible to do this like you describe, but maybe that this is an idea to handle your problem:
Put all IP's in a file and if a user with one of this IP's have logged in he get this pop up with the new message. After the message is given, set a cookie to the client that he didn't get the message twice.
Of course there have to be something to check the IP's in the file but I think you can handle small functions like this...