OK, I tried my script. It worked and displayed the critical battery screen as expected. Pressing the power button or plugging the USB into a power source (wall or computer) boots it with settings and documents intact.
I also tried a script with sync then halt and that worked similarly with differences below.
Code:
#!/bin/sh
sync
halt
results in a boy under the tree screen with the caption:
"Please wait a moment while
your Kindle starts up."
In fact, it sits there powered down, the desired result.
As to my script, because there is a symbolic link from writable
/var/local/eink/images/critbatt.png
to
/opt/amazon/low_level_screens/critbatt.png
I thought that I could replace /var/local/eink/images/critbatt.png with a custom image, which I could, but it was not used. I am guessing that /opt/amazon/low_level_screens/critbatt.bmp.gz is waht is actually used.
As might be expected, halt and poweroff are symbolic links to reboot, but POSIX executable know how they are called. In theory, halt stops the CPU, which continues to drar power, while poweroff actually powers the system off. On the Voyage, I can not tell the difference or see any meaningfull difference in dmesg output.
sync followed by poweroff has no path dependencies and seems the most portable across all systems. My original proposed script is at the mercy of Amazon not rearranging things.