How do I use an animation curve from the parent skeletal mesh to affect a child skeletal mesh?

I have a hip holster asset on my character that has some shape keys to help with clipping during locomotion.

I added an animation curve to the locomotion animations that drives the shape keys for the holster mesh. But it only works if I play the animation directly on the holster skeletal mesh.

But the anims are playing on the main character mesh and the holster is a child, hooked up via the leader pose component. So it doesn’t deform because I’m guessing it can’t “see” the curve from the animation playing on the parent mesh. It just follows the pose I guess?

How do I get around this? I want animation curves playing on the parent mesh to drive shape keys on a child skeletal mesh.