I have a base body and make clothing for it, but in the game we use an item mask (just a texture of the body that's white, but black underneath the clothing) to clip out the parts of the body underneath of the clothing. Currently we just make it manually by painting black on the body texture where the clothing is. Under the assumption that it is skin tight, how could I go about using the holdout material (or if something else is better) that every part of the UV is blacked out that is underneath the clothing. I looked into vertex paint and a few UV projects but wasn't able to figure out how to go about doing it.
You'd have to invert the image after to get it to be black under the clothes since the bake will use black whenever it's rays don't hit anything.
If that doesn't work maybe look into dynamic paint, you could probably accomplish something similar with that.