Is there a way to implement this rig logic in apex/kinefx?
I've been messing around with apex but I am breaking the rig of course, because I can only imagine ways to do this that create dependency loops... aaaaand, I'm also not a rigger.

if you want it as seperate control, then, im not sure if it is possible, it would most likely create cycles in your rig.

Here's a kinefx example. I'll upload an APEX example later. Also, I dig the music track in your video.
So basically, I'm setting all the joints under a single root joint and all the other joints have their logic set up in a rigvop. It's not perfect, but the logic here is pretty simple; it's just a series of blended transforms. A similar concept can be applied using APEX.

tried to make this, and if you don't unplug the promoted transforms from the transform objects, you can use the values in other nodes too! so you don't have to do this later.
I hope this is what you want to achieve

That's awesome, Umang_Raj beat me to it. I'll post my APEX version as promised. You have several options now. One thing worth noting with my setup is that it uses the Skin Controls feature, so you'll have to make sure it's enabled in your viewer state settings by pressing "p" in the viewport and making sure "Use Skin Controls" is checked:

So nice!
All solutions look great... with one small caveat!
How would go about solving for local orientations. I am having trouble getting correct world positions and also assigning them back.