CAT Mirror animation with constrained objects and IK

[Nixellion]

Hello.

So, I work on animations for a game on Unity. I made a bunch of animations, and once they reached our boss, he said "Awesome! I love those. But I want you to mirror them."

And I'm stuck. I really don't want to just re-make each animation from scratch mirrored, which will take time to get them exactly as they are, just mirrored.

I tried using Cat's mirror tool. It seemed to work, at least after Euler Filter, but...

A character is holding a hammer. It's rig consists of 2 bones.

Weapon0 - attached to palm and constrained to weaponCtrl bone. weaponCtrl bone is attached to nothing and constrained to the chest bone. 

And they are animated. And I get a lot of weird motion after that. 

So, I wonder if there is a solution to mirroring animations, maybe with scripts, maybe with a few passes, but which will give me exactly same animation but mirrored. 

Thanks