can i make a mask affect one object

hi guys, this is my first post on here, my problem is: i'm animating a screen with a big hand holding a sword putting down to the ground, exactly like this:

(from 13:01-13:04)
can i make a mask that only hide the hand&sword but not the background, then i could slice down that mask and they slowly appear
i can make this effect in 3ds max & edit it on AE but in Unity this is impossible, i searched on google but nothing match my question
hoping for some help !!
ps: pls pardon my bad Eng, and i only new in Unity, about 2 weeks

 

Pay attention to model 2nd uv set and mask texture setting (clamp, alpha from grayscale).

 

thank so much, thats it