A great idea, but be aware that aerodynamics does funny things. Unless the tube is actually in the airflow creating a suction, you might find the flies buzzing around in the cabin. Just because the door or window is open doesn't mean that something will get sucked out. More than one pilot has landed with Aunt Flo's ashes scattered around the cabin.
If you're letting flies out, is it covered by the dropping of articles reg?
You can't "drop" a fly!