Adding light behind the tile would be very cool, but I'm not sure how to approach that in this situation. Obviously the installation of the tile would have to be attempted in a different way - not your standard thinset. Instead of the tile though, you could use a removable piece of frosted glass with lighting behind it.
A slightly different approach, but easier to achieve, would be a recessed slot at the top of the niche that hides a wet-location rated LED strip for example. This would give you nice indirect accent lighting for the niche. I hope this helped...