Not sure this is possible easily. Would be curious if anyone has another solution.
What I would do is create 2 videos, one that is the loop and another that is the loop plus whatever you want to have after it.
Have the loop on a higher layer than the longer file, and have both files at full opacity.
Set the sync id of the longer one to match the ID of the loop. Then you would have to be watching and have a preset to change the playback mode of the longer one to not be a sync ID, and play forwards and then fade out the loop. Theoretically this would work.