This is an old trick, but always fun. It's easy to change the status message on a networked HP printer; just telnet to the printer's IP address on port 9100, and type the following:

@PJL RDYMSG DISPLAY="YOUR MESSAGE HERE"

Press enter, and you're done; the new status message will stick around until you change it back, or someone restarts the printer.

Feed Me A Cat

If you want to put some effort into it, maybe make an agent to change the message at a random interval, you can also write an application to accomplish the same thing. See here for an example in C#.