After wrecking my brain for a couple of days trying to replicate Jeff Brodsky’s Variable FK trunk rig and failing miserably, I now finally managed to pull it off.
Since I intend to use it for a spine I only added a single control but now that I have it assembled like this, adding additional controls is trivial.
Thank you also to Sergio Mucino for his walkthrough on how he set up his version of it in MODO. Good stuff! 🙂