Perhaps the reason I cannot find any of the useful lab126 custom ioctl stuff in the gpl code is because there is a large folder full of PATCHES, and a script to apply them... Maybe the code would make more sense if it was pathched to match what is in the kindle... ?
EDIT: No, those patches were already applied. They are for patching the generic kernel code if you download it. Downloading 5.1.0 source now, to see if diffs that could break the ioctl(), or at least what changed... Perhaps one of their patches makes it incompatible with the freescale reference manual...