What I would do is create a .cfg file that does nothing more than turn on your backlight. I would then name this config file ZZZ.cfg or whatever it takes to make it the last file displayed in your file browser. Copy the .cfg file to the root of your player, and then you can easily select it by scrolling to the bottom of the root file tree (which you should be able to do quickly and blindly) and selecting it.
Another similar option (requiring only two button pushes) is to create a folder in your DAP's root named "#Backlight" (the # so it sorts to the top) and place the backlight .cfg file in that folder. Then upon booting and desiring the backlight you simply need to press your player's action/select button twice.
Of course, these suggestions ASSume you are browsing by files/paths and not Tag Cache, and assume you sort alphabetically, but hopefully if my assumptions are wrong you can benefit from the concept.