How to create a mask from texture for post-processing material?

Is there any way to Mask out Actors from postprocessing without using “custom depth stencil” method?
Ideally by just using an alpha channel, which is used from the shader of an actor?
(like know features from standard render passes in other software)
I have flat objects, easy to determine by their contained alpha channels (image planes), but I need to get a mask out from these alpha channels, which I can use to mask for the postprocessing material.

Other methods do not work for flat objects:
My main problem is, that I need to get some actors out of the post-processing, custom depth stencil is not working for them!!

Thanks for help!