How can I create an ADSR envelope to control the output volume of a track in Ableton, which is triggered by the audio of another track passing a certain loudness threshold?

I want to create a M4L audio-device which acts like this: It detects when audio on a certain track has crossed a "user specified" loudness-threshold, and then starts increasing the loudness of a second track, the target-track, from zero (silence) to a specified maximum volume, over a specified length of time. The action would be similar to that of a gate, except that it would be detecting audio on one track to affect the loudness of another -- and it would open s-l-o-w-l-y, like an "attack" portion of an envelope set to upwards of a full minute.

The use-case is this: I'm trying to create an ambient drone which, as a key is depressed and held, gradually evolves and adds new layers and textures, automatically. The first layer would gradually get louder and expand in stereo-width. As it does, once it crosses a certain loudness threshold, the next layer will slowly fade-in. And then, gradually, another layer, and another. Each of these layers is actually a different instrument in a different track or chain. Once the key is released, each layer should fade-out according to other programmatic settings. And I want this all to work live, so to speak. Using manually-created volume-envelopes is the antithesis of what I'm trying to do. I want this ever-expanding drone-note to essentially, in the end, function like a single instrument.

Anyway, it seems like M4L should be the right way to approach this problem. I feel like what I'm asking for is essentially an ADSR envelope which can control the output-volume for a track or a chain in a rack, and which will hold on silence until it is triggered by the volume from another track rising above a certain threshold.

Does anyone have any idea how such a device could be created? Or do you know of a device which already does this? I have to confess, I'm a complete noob at M4L, and this is the first time I've had a specific idea for a device I wanted to create. I've only really started to dig-into whatever documentation I can find, but honestly much of it is written in a kind of shorthand that I can't make heads or tails of.

I was trying to use the adsr~ device in various ways last night to accomplish this, but I never could get any audio to pass through it, and eventually I decided that I really don't understand how that device's connections are supposed to work.