So, there you have it. While the hard disk controller is a beast without much data known about it, it's still perfectly well possible to reverse engineer it and to write custom code for it. The unknown-ness of the controller does make it harder to write generic hacks, which makes me doubtfull that a thing like the evil firmware patch will ever be seen in the wild: it's much easier to just get another zero-day software exploit than reverse engineer the firmware of every single hard disk every server you stumble upon has.
I also hope to have proven that a broken hard disk is something you can still use. While the mechanics of a broken HD probably are shot, the PCB still contains an usable embedded system, which actually is pretty powerful considering you can usually get broken hard disks for free.
Releasing the source-code for a security project always is a nasty subject. I want to release code, but I do not want to be responsible for a lot of permanently hacked servers... I decided to compromise: you can download the code I used here, but I removed the shadow-replacement code. Make note: I'm not going to support the process to get all this running in any way; it's a hack, you figure it out.