How the low battery message graphic is loaded?
I believe it's just an early stage of the bootloader that checks to see if the battery has enough charge before going on to boot iOS. It's no different from being able to display the Apple logo while iOS is booting.

If you download one of the IPSW files (iOS images), you can unzip it and find several images inside - applelogo, batterylow, glyphplugin, and so on. A full listing can be found on this page: IPSW File Format, and I also came across this page which has instructions for replacing the low battery image with something else (essentially just replacing the file inside the IPSW with another one in the correct format).

That would seem to indicate that iOS 7 will (or does) have a new version of the image for all devices, just like it has a flatter Apple logo at boot time.