When you use a texture of 8192x8192 and then use the same size for the mask layer you need a lot of memory. So maybe, in this case, the mapping idea is not bad ...
In between I was able to use this PBR shader (I won't stop learning ... ) ... but as I thought, it does not make too much sense, when your skin allready has some makeup from the original pictures. And all my models have this "feature" ... In between I've done a less complicate method to create tattoos for the "alternative models" scene (see gallery). These tattoos should look a bit amusing and not like the Gothic stuff ... so a method simply multiplying two textures was okay for me.