I can run through the idea for you, I haven’t done this I am just thinking about it, obviously this is not ideal, but it might work and is fun to just mess around with the ideas - it might be nonsense
Incoming gate triggers first sample slice as a one shot
The first sample slice includes the middle portion you want to loop
Using a copy of the incoming gate insert a fixed delay (set to the same length as the attack portion of the sample) and convert to trigger. This delayed incoming gate triggers the record action of the looper.
Using a second copy of the incoming gate with another delay (set to the length of the attack portion plus the length of the section you want to loop) and convert to trigger. This longer delayed incoming gate triggers the play action of the looper - the only audio you should now be hearing is the looper because the first section of the sample has stopped playing
Finally, invert the incoming gate, or detect a downward slope, but either way output a trigger when you release the key. This will trigger stop on the looper and at the same time trigger the second slice of your sample, the decay portion.
At some point in this process you would need to shift the sample slice select parameter up one.
Hope this makes sense
Interested to hear how you (or anyone else) gets on if you try it