I wont call this a hack because it's not, more of cute gag. Most of the newer model HP printers come with a LCD screen that gives basic information like "Printing" or "Ready". This just allows you to change it to what ever you want.
To change the screen just open a telnet session to the device on port 9100 and use the command:
@PJL RDYMSG DISPLAY="YOUR MESSAGE"Then press ctrl+] or ^] to escape.
That is it, enjoy!