This is totally not my area, so bare with me if I talk bullshit:
Having never really used Looper, but just looking at the problem I'd say:
1: If it wouldn't be live, this would obviously be done with automation
2: Live, I would use clyphX to achieve this, as you can pretty easily create action lists that you want to happen by triggering a clip or just by triggering a function (something you can set up in clyphX).
viewtopic.php?f=1&t=153113&hilit=clyphx
(For instance: Raise the Send A volume slowly to 0dB over 4 bars after triggering the Xclip, and then drop it to -inf upon stop when you the stop of the Xclip is reached (or you stop it manually). And much much more!)
If no one else has a more straight forward response to this, you should definitely check it out!
It's free, easy to learn, totally stable and almost as powerful as M4L for many things (except building instruments of course)
Or maybe you could even just set it up with dummy clips?!