Wow, @DomesticExtremis, You did your own research.
Probably you're right on the various low level bits. I hadn't time to check. My suggestion about UVLO was just after a quick read of the datasheet, but given there's an specific error for that on kernel sources I would came to the same conclusion as you: not our problem.
Please keep in mind that the PMIC you're exploring is just for driving power to the screen and shouldn't have the capacity of powering the whole system down. There is another PMIC based on MSP430 that's driven via i2c and that handles the various bits exposed to linux (battery, frontlight, GPIO...) and it is the one that could reset the SoC on demand.
https://github.com/pazos/linux-2.6.3.../mxc/pmic/core
On my Kobo Aura HD it has its own serial header (labeled J2) but it just produces gliberish data connected at standard baud rates from 9600 to 115200.