Posted a thread about this a while back.

The low level screens appear to only be used during specific parts of the boot process (i.e. between the first splash screen and the one with the loading bar, or the transition that happens when you plug in a completely dead kindle).

So far, what I've found works is the png files in /usr/share/blanket. For the text on the splash screen (and some other stuff), check the /usr/share/locale/$YOUR_LANGUAGE/LC_MESSAES. Inside you will find *.mo files which you will need to convert to *.po files (personally like this converter) and then edit them with a program like Poedit, and then convert them back to *.mo files and transfer them back onto your device.

I highly recommend renaming original copies of whatever files you edit so that you don't lose them.

Before you can edit any files outside /mnt/us you need to enable write access, use USBnetworking as knc1 said or you could use kterm as well.

As for serial access, it is highly unlikely that you will break anything if you only stick to editing the .png and .mo files and use common sense, although I would highly recommend installing USBnetworking as well as the (Coward) Rescue Pack anyways.

There's still a few screens that I don't know how to access, although if the Kindle functions anything like an Android device it's likely that the pictures are literally encoded into the bootloader or some other file that isn't directly accessible from the flash storage. There's plenty of threads here talking about a bunch of low level hacking that's way over my head but in theory there should be a way to open this file in a hex editor or something and then just copy paste the raw png data over the original and flash it back. A