Hi All, so to save some stress and trouble - YES, YOU CAN ALWAYS LOOK UP SERIAL NUMBER ON THE DISPLAY OF THE HX STOMP. What you need to do is:
-switch off your HX Stomp XL
-press and hold encoder 3 (the third knob the right under the display)
-switch on your HX Stomp XL
-you should see the serialnumber displayed
This is what I got from Line6 support and I just tested this solution as I was missing the serial number sticker as well. I assume this works the same for Effects and regular Stomp.
Happy product registration and shopping for discounted Native plugin! :)