Having control over the advanced output is definitely a great thing. A lot can be done with the slices and screens, We might mix a few layers to a screen and to do a fade out of that screen without effecting the other screens which uses the same layers will be possible if we get controls over the advanced output.
There's really no reason to request this.
Everything you need can be controlled from within the main UI using Slice transform. Here's a great video that goes in depth on how to use it:
Further more, You can assign Layers, and Groups to specific slice in the advanced output, so your other screens won't go black when you kill that layer or group.
In regards to color correction or changes for each screen, again, just assign a layer or group and then ad a color FX to it.