Gravedigging,
For about $5 (or less), you can totally turn off the backlight, without needing to do any sort of hardware hack, or rooting the firmware at all!
1- You can disable the screensaver, by typing "~ds" in the search bar (disable screen saver). This will allow the Kindle to remain in operation, even when it should be in sleep mode.
2- The $5 will buy you a magnetic strip like this one:
https://www.amazon.com/Roll-N-Cut-Fl.../dp/B005HYDC68
You probably will be able to find fridge magnets at an even cheaper price,
like these ones:
https://www.amazon.com/Kikkerland-Ow.../dp/B008VE62E0
You turn up the screen brightness, and test the fridge magnet on the bottom right corner of the Kindle. Depending on the location, direction, and orientation of the magnet, it will either work, or appear to do nothing. If it doesn't, try rotating the magnet, moving it around a bit, or reversing it.
Once you found the magnet's best location and direction, take a scissors and cut out a small square, about 1/2" sq of the magnet.
You can either glue it on the kindle on the exact spot you've tested it with, or just place it in between the kindle and the cover.
The backlight will be turned off, but you're still able to use the kindle like normal.
You can also remove the magnet, to enable the light again.
For this reason, I think it's a better solution for those that want a more 'permanent' light off-solution, with still the ability to resell later on; and without the hassle of difficult hardware hacks, that may void warranty.
Battery savings?
Depending on usage and settings, you'll see a max savings of round about 50% of useable time (24/7 usage), and about 5-10% of battery savings in standby and occasional use with Wifi off.
Wifi on, and mostly in standby, the savings are negligible.