Hi Chris,
Doesn't appear to be any consistent way of entering JVC service mode. Here's another way for older JVCs:
Simultaneously press and hold SLEEP TIMER, DISPLAY and VIDEO STATUS.
A few others: Press and hold MENU and INFO, press and hold DISPLAY and PICTURE MODE, display the menu on the screen, then press INFO and MUTE.
Must have given each designer short shrift after he completed a design, then hired a new one. Bruce