I've been digging around threads like this one as well as the /opt/amazon/low_level_screens folder, and as far as I can tell, the Kindle seems to reference the /usr/share/blanket folder mentioned in the thread above more often.

Editing splash.png in /low_level_screens seems to almost work, but the image is only shown for a second and it doesn't render correctly(gets stretched beyond the boundaries of the screen)

bg_default.png in /blanket only seems to affect the "Please wait while your Kindle starts up" screen that happens right after reboot and the screen with the loading bar. However, it appears that the actual text on the reboot screens is stored somewhere else since the text still shows even after replacing the image.

Looking through some of the other images in /blanket also seems to indicate that the text for those screens are also stored elsewhere.

Does anyone with a better understanding of the Kindle filesystem know perhaps where I can find these text files or perhaps even other folders with boot screen images that I can edit?