There was a guy during the first GBA days that put his name on the map, buy selling a "Back lighting" kit that you could do to your GBA fairly cheap. The site was "Tritonlabs" I think, but now he has just made that site into his "Triton Labs" music site. And there is no more info on it.
Point being, years ago on that site you could purchase a backlight kit for GBA systems and Neo Geo Pocket as well.
So yes, it's possible to mod the NGPC to be backlit.