Add World Rotations to Bone's Local Rotations

Question

It's pretty easy to combine local rotations from another bone onto the active bone's rotations using "add, local space, local space" when the other bone is using a similar orientation. But is it possible to combine another bone's transforms in world space without losing the active bone's existing local rotations?

Essentially, I want to perform the same operation as "replace, world space, world space", but while keeping the exact rotations of the active bone (replace destroys them). You would think "add, world space, world space" would work, but I get something completely different. I'm assuming because Blender is trying to convert the active bone's existing rotations to world space before adding (multiplying) them together. And of course "add, world space, local space" won't work, because its trying to apply world rotations to local axes".

In my case, the bone I'm copying rotations from actually does not rotate in its local space at all - it is only rotated through its hierarchy. This is why I'm forced to use a world space copy.

I've tried every combination of mix modes with "world space, world space", but nothing adds the world space matrix to the existing local space matrix. Is there a way to do this without writing a custom script?

Edit: Just to make sure my situation is clear, I could achieve what I want by copying the active bone's local rotations to something else first (temporarily, to cache the data). However, I don't think something like this is possible, even if I wanted to:

1. Copy active bone's local rotations -> temp bone local rotations (replace)
2. Use "replace, world space, world space" as mentioned to get desired changes
3. Copy temp bone local rotations to active "add, local space, local space"

Answer 1

If you want a bone to copy the world space rotation of a bone, before its own rotation, simply sandwich in a parent with the constraint instead, a parent with the same axes as the target:

If your question is, how to do this without a mechanism bone, you don't. You use a mechanism bone. It's not a big deal to use a mechanism bone. You don't cut down trees with herrings, and if somebody asks how to cut down a tree with a herring, the answer is to use an axe instead of a herring.