Hi supermag
What you seem to be describing is having something that works like the classic Maya ribbon spine set up. In Maya this would involve using a NURBS surface instead of a spline to create a robust and easy to control twisty stretchy rig. Ideal for spines long necks and tails and tentacles.
There were other methods as well that created the same effect using a Follow Path constraint as you describe. I re-designed and adapted a Follow Path spline rigging method I had used extensively in production in 3DS Max for Blender, Although this was quite a few years ago now when I was learning Blender for the first time and testing things out. It uses a normal Blender Follow Path Constraint.
This method I tested out by rigging and animating a small feathered dinosaur. It worked really well. I used to have a sample file link for it on my signature but it has long since expired. But I also think it needs updating now as the rigging in Blender has had so many updates since 2.8. This was made in the 2.7 series.
One of the Blender rigging features that received a lot of work since then though is the property’s of the bones themselves. In particular the squashy stretchy B Bone features. These can create a whole multitude of twisty stretchy rigging possibility’s now. My advice would be to do a deep dive into Blenders unique B Bone rigging possibility’s before trying anything else.
Edit :
Hey a quick update. I found the original demo Blend file. It works great in Blender 2.79 B. As for Blender 3. It opens OK and mostly works. Everything seems to be in place but the twsit motion and control is broken somehow. I suspect this is likely a multiple transform that has appered or a dependancy issue due to the re-writes on the bones and IK systems. I might have a look when I have time or if anyone else has the time to trouble-shoot it please feel free.
Toka_Spline_FK_IK_Demo.blend (564.9 KB)
All the best