Constraining bone to twist opposite it's parent (for upperarm_twist_01_r)

I’m trying to get proper twist with an IK rig I cam creating. upperarm_twist_01_l and upperarm_twist_01_r are supposed to rotate opposite their parent (based on this: A.R.T Trouble testing the forearm roll - Character & Animation - Unreal Engine Forums)

I want to get the delta rotation of the upper arm R bone from the ref pose along the bone axis, and apply the opposite of it to the upperarm_twist_01_r bone.

I tried with a bone driven controller, using a -1.0 multiplier to try and reverse the X rotation:

but it doesn’t seem to work correctly:

From reading the code, copying X copies from a Euler rotation, so I think I might be getting Euler angle related errors, since I think UE4 applies X last.

Is there a different approach I can take to solve this? I haven’t been able to find the right nodes to do what I need.