There are some tools out there that allow you to remap keys, like the Rhodium Keyboard Controller over at xda, but unless the action you're mapping has a hard key / function / combo key command to start with it's not going to be mappable. It doesn't appear as though there is a way to change the backlight / brightness except for the Settings | System | Power applicaton. Therefore I don't think you're going to be able to map that to a hard key.
You could always buy a Treo Pro...they are able to do this out of the box
