I have found a workaround but i had to use two devices.
Basically, I built a 'Sidechain' Device where you can switch on/off VSTs when audio is detected or not present......
First I got the Amplitude data using the L.O.M and Live API from Abletons Track Level meter, but it's got this strange hold/fade thing where once the audio has stopped (instead of shooting straight down) It slowly fades down.
If a live.meter is attached to a plugin~ object it responds correctly to the incoming audio. But it takes arguments & can't be configured to be track specific, so i had to build a seperate device, with Sends that has to be put in the track that you want to sidechain from.
just wondered if there was a 'one device' solution