Why not just adjust the buss level? It seems like what you want to happen requires every channel compress the same amount regardless of their summing levels, which would only be possible by pre-fader sidechaining every channel on that buss to the key input on your buss compressor, which sounds like going around your *** to get to your elbow to me. A VCA-controlled summing level would do the exact same thing as changing the summing level to begin with, and your buss insert would compress more or less as before. I don't see how this is possible as described, or even necessary.