I don't know how tech savvy you are (or how well heeled your clients are), but I instantly thought that some of the new wireless power technologies, like
eCoupled, paired with some LED bulbs, could get you lights that worked if they were on the right place on the shelf.
If I were chasing this down, I'd see if I could get some sample coils from
Leggett & Platt - eCoupled and put something together. Heck, if you play it right you might even be able to take this to them as a product need.
Oooh, here we go: the
WiTricity Developer's Kit, comes with an LED light, assumes the antenna 15-30cm from the receiver. If the LED isn't enough, it looks like it comes with a couple of "Wireless energy capture modules". If you can't source enough current to get as much light as you'd like, maybe you can just assume a lower duty cycle (ie: the lamps are only on a few hours every day) and charge batteries the rest of the time that the lights use when they're on…
(Yes, I am a geek who's willing to go way over the top to find absurd solutions to problems…)