Procedural animation after animation interpolation.

Hi , guys, I have been studying and learned a lot more on ue4 animation mechanism. I watched the UE4 tutorials about persona etc.
The thing I found got closest to what I wanted to do was this tutorial:

around 12:53

The mentor scales the head bone, the only problem is that the only way you can get the bones matrix is inside a blendspace, in that case idle/run blend space. I would like to have this procedural animation work on any animation blend space, not just specific one (or replicate the setup on all blend space).

I am trying to get it done on event graph animation blueprint, but the only thing I can do with bone is get bone name from an index, or get index from bone but not the actual matrix/transform.
Anyone has any clue on how that can be handled? Right now I am trying to get it done as blueprint just as a prototype.

Here I have a prototype of muscle setup in maya:

The goal is to attach some bones on the muscle to get good deformation, then at runtime to jiggle those bones to get some secondary motion.

Any tips on how to get the matrix bone after interpolation would be great.