Minor update on how things are going:
I managed to find some time between work and life outside Kindle hacking and started looking at a couple of optimisations that will make life easier:
- K3 Flashing - flashing a rootfs for this device takes a longggggg time (read: several hours) due to the k3flasher method being used; it's not efficient in the slightest so fixing this was a high priority. After much digging around, I was finally able to build a kernel that would boot on this device and use the g_file_storage gadget from initramfs to export the entire eMMC, where it can be flashed with a rootfs at a speed that matches the rest of the devices
- K4NT/K5 Flashing - same as above WRT g_file_storage; this device doesn't take too long to flash anyway but the advantage of doing things this way is that you don't need to go through the whole "enable SSH from diags" dance that always struck me as a high friction process
The research needed to build kernels for these devices has paid off in other ways too - it took a while to figure out how to actually get anything to boot on the older devices (solution: scratch-built Ubuntu 10 container + ancient CodeSourcery toolchain) but now that's out of the way, U-Boot + Kernel builds (w/ custom initramfs!) from source are back on the menu.
This means that there's eventually going to be a version of Popcorn jailbreak for these devices too, which will be especially useful for K3+K4 as I've always found those devices to be a massive pain to jailbreak using other methods.
Finally, I've also been playing around with Barebox on K3 and K4, which opens up fun and interesting possibilities. It's significantly nicer to build, is much more powerful than the ancient U-Boot version used on those devices and opens up another method of flashing/jailbreaking those devices as it can export the entire eMMC without bringing up a kernel in the first place(!).