If you're any way inclined toward modding, or know/have a service/guy that'll do it, there's always the afterburner kit. SP screen into an original GBA.
http://www.gameboy-advance.net/accessor ... _light.htm
Good luck hunting one down, if you choose to go this route.
There's also a series of clone-GBA consoles, that have a backlight. There's a link to buying one in the expanded-description of the Youtube vid below:
They come with SD carts slots and emulator and whatnot. Have at it, I suppose.