Can you define punishment? Leaving backfill is exactly the same as leaving normal game. I leave backfill if it is into Flash. There is only a penalty if you have already left games previously.
As I said, leaving backfill IS EXACTLY THE SAME as leaving a standard game after assembly. There is no difference in the way they are treated for penalties. I have done this many times for backfill into flash. Because I generally don’t have many left game I rarely get a punishment, and if I do, it is 2 mins.