Well, I won't throw away my device, I will always be open to new propositions
I also have unpacked 5.3.0 firmware and I'm looking for anything I could use to at least perform some manual fiddling with files on rootfs, but without any luck yet. The only idea that came into my mind was to use custom fonts handling from /mnt/us/fonts to exploit some libfreetype vulnerability (it seemed to have problems with segfaults on some custom fonts in older firmwares), but that would be real, assembly-level hacking and I lack proper knowledge to do that.
But I'm still trying.
PS. Anyone knows how Amazon's ToDo service works and what it can do? I wonder if some kind of self-man-in-the-middle attack between Amazon and our Kindles could help with forcing Kindle to do something. For instance, if only we could put some file in /etc/upstart, then we could run any script from /mnt/us on boot.