Animated 2D mask ??

Hi guys,
I’m working on a 2D action platformer, the animations are drawn frame-by-frame, not using the unity animator, this is why I’m not looking into Animation Layers.

So, what I’d like to do is find a way to mask the lower part of my plaer body to be able to do upperbody actions while stile running (keep on playing the walking/running animations while the attack animation is also playing).So far, I haven’t found any possibility to do so…

What I was thinking is a kind of “mask” who could hide the upper part while I’m running a second animation at the same time. Maybe duplicate my running spritesheet and erase the upper part so I can use it as a “animated mask” ? Or maybe a tricky shader ?

Do you guys have any idea of how I could accomplish that ?

I too would like to know how to this for my game as I am trying to do the exact same thing, so far I haven’t had any luck.