“exclude this object from [this postprocess]”
^ No go for now (and probably forever)
It’s technically possible but you are opening a can of worms and you often need to understand exactly how renderer is working (Unity custom pipelines, Unreal Engine custom depth pass, etc).
“can I paint [my_channel]” (emissive here)
In the future but not now, I already explained elsewhere that it adds extra memory and I need to rework things so that unused channel doesn’t take extra memory.
Otherwise many user won’t be able to re-open old files with lot of layers.
One of the reason why I added additive is that it doesn’t require face ordering, meaning if you have a very complex mesh, it will shows correctly in the viewport (unlike blending, where you’ll see some weird patterns). It’s like that in every real-time renderer.
Also if you are using Bloom you should almost always use Tonemapping, otherwise the brights pixels will be saturated