torne: I think you misunderstood the OP's request.
The problem is on some players you don't need the backlight during the day. At night you do need the backlight, but are unable to navigate menus in the dark to turn it on. This means you either waste power in the day, or you can't use the player at night, without a source of light to enable the backlight.
He wants some type of hotkey, or series of keypresses to activate the backlight, that can be performed from any menu etc.
Simply turning on the backlight when any key is pressed wastes power during the day.