Thanks, that's really helpful! I managed to find some blurb from a 2008 post on another forum:
"Made from 3mm acrylic which is 1 mm thicker than a standard scrachplate with a full metal backing to prevent any interference, it is easy to change them over, powered by a 9 volt pp3 battery (which is included in the price) and can be hidden in the control cavity under the plate in most Strat type guitars. These plates are not back lit , all the components are in the plate (except the switch) , giving you a light all around the plate and screw holes . The plates come in a variety of colours , Blue , Green , Red , White , Amber or a combination of 2 or more . And if that's not enough we even do one that flashes ( click the pictures to see our full range). No need to cut or route your prize axe , no need to solder any wires (this only applies to most strats and P basses), simple to install . The light shines out from the edges and can be clearly seen from all angles."
I imagine that the edges are outlines and the actual opaque pickuard is a top veneer so the lights are in a sort of hollow area. I'll play with some perspex pieces and LEDs to see if it's feasable at home.