Hands IK and View/Camera rotation

I’m currently implementing a IK solution for weapon/arms animations similar to this.
In short: one weapon bone that’s not in the main skeleton hierarchy but moves independently. The weapon is attached to that and the hands are moved to that bone via IK. That means I only have to move that one bone in my anim blueprint and the arms/hands follow to always align nicely.

Problem: This doesn’t work for vertical camera movement right now, the weapon just stays oriented forward. I’m not using a separate first person mesh so I can’t just rotate that but need to do something else.
Now, the question is: what?

I can move some spine bone but that only moves the arms, not the weapon bone so it isn’t really viable. I guess I need to calculate the location/rotation of the weapon bone depending on the camera rotation?
Any other ideas or a proposition of how to do that specifically?