Idea: Non-Destructive Slice Decay'S

Not what you want, but you can get similar effect through using a couple of scripts.

First slice your loop in the sample editor and make sure you have the original sound beat synced to the correct value,

…than use Dblue’s slice to pattern script to get the slices translated to events in the pattern editor:

Next move: go to the automation editor and run another script of his to get automation from notes:

You can link the note lengths to whatever parameter, not only a gainer.

If there is not enough resolution between notes you could expand the pattern first, for example using this script: