I just remembered something.
I have many old Kodak cameras which all of them have those holes on their back. This includes Kodak Tourist, Six.16, box brownies, etc, etc, etc. Some of them no longer have the red plastic thing or the cover. I use re-spooled and adapted 120 film to use these. On those that didn't have red plastic or the cover, I just left it and loaded said 120 film, covered the hole with black electrical tape, and went shooting. Obviously, I had to remove the tape to wind so I know when fresh emulsion is aligned to the next frame. This was done in daylight.
End result was, I saw light leaks (well, these are old cameras) but nothing attributable to the hole on the back. Nothing circular or in the location where the hole would/should be. So this leads me to think, today's back papers are thick and light proof enough for brief exposure to natural light. It may not fare so well if it was wide open and facing sun for long time.
Just thought I'd report in.