Did you try pointing out to the customer that it's totally worthless and then removing it? No one who is accessing the system illicitly is going to read it and say "well, gosh, I guess I better cease my nefarious activity right away". The majority of people probably won't bother to read it, even fewer will read all of it if it's long enough to require a scrollbar.
Code:
$ zenity --title="pointless message" --text-info --filename=pointlessmessage.txt
where pointlessmessage.txt is a file contain whatever text needs to be displayed.
What if someone logs in by hitting ctrl-alt-f3 then does a non-graphical login. Do they be shown the message, or are such logins not subject to the same conditions?
If you put the blah in to /etc/motd it will gets displayed to people doing non-graphical local logins and ssh (if that's allowed) too.