Virtual Server Mixer Mode to background

I would like to see a mode where the Mixer mode of the Virtual server not only works with other video/effects layers, but also with the levels in the background.This would make it possible for a server layer to Lighten, Darken, Dodge, Burn, etc. the background colors/intensities of the fixtures, that are recorded in a cue. Now it seems that they can only replace or add to those background values (HTP), despite having the crossfade parameter in the virtual server channel.

I was hoping to achieve the following: make a mask for certain pixels with a video/effect layer, in order to make those pixels black.