Rendering Decals in a seperate pass?

I would like to render decals in a seperate pass like it is possible with the Custom Depth Pass and Seperate Translucensy.

It they can’t have their own pass, It would be good enough if they can be rendered using the Custom Depth pass or something like that. Basically I just need a mask of the decals in my post processing material.

Is there any way to do this?