The difficulty is you can't see the film or the magazine to load it in. You have to do it by touch. You also need complete darkness, which is why film shooters use the bag. Could you do it under a poncho? Sure, if you can get into it and keep it dark inside. The least little light on the film will expose it. The PG film does not show any areas that may have been exposed to light, unless it was edited out.
I doubt a pro would trust a poncho or any cloth that may let in some light. An amateur may not care and take the chance. However, If I just shot the first real clear footage of a Bigfoot in the wild, I wouldn't trust screwing it up by diving under a poncho to change the film.
Also in a bag if you drop it there isn't a chance it may spill out and get exposed to light. Remember you are fumbling with it in the dark. It takes skill.
In my opinion you would be crazy to change the film in anything that could possibly cause it to be ruined. But that's me. If Roger rented a camera and had no real clue how to properly use and handle a camera like this then yeah, he may have been crazy enough to risk it. Or ignorant.
Again, I'll go back to the point that he was shooting a documentary. He intended to make money from his show. I assume he had to have some basic training before attempting such a project? I have to assume he, at the very least knew how to handle a camera, and changing film would be a major "Film 101" topic.
Keeping in mind he was not up there shooting a home movie. If this was the case than a little 8mm would have been easier to handle, the film is in a cassette. 16MM was considered a lower end pro system. You had to have some skills to use it.
So that is my question, how dumb was he? Or did he have any training. Or did he have help?
If he did change the film under a poncho he is one lucky fellow, a multitude of things could have gone wrong that would have resulted in us never seeing that film. What if he dropped it? What if the wind blew the poncho up? What if he jammed it and needed to take it out, lay the film down, and fix the camera? What if the film got stuck and had to be re-threaded? You need to be able to carefully change the film. Plus you need a clean environment to work in, a bag is usually clean. A poncho?
Not to keep driving this point, but they are in the woods on a sand bar. Sand gets into everything.
Grime and dirt or fibers from the poncho may concern me even more than light leakage.
I will say this. If John Green was a semi-pro or a pro, and had entered film festivals with his projects, I seriously doubt he would change film under a poncho. Especially a film of this importance. You just wouldn't do it unless you are so familiar with the specific camera you use that you feel you can get it changed with no issues. IF John Green was there I don't think he would chance it. However, he would likely have the skill to pull it off.
There are pros that are very confident with their equipment and can change film quickly and under difficult situations. I cannot fathom that Roger, with a rental camera, and little to no field experience would be so confident.
Ask any wildlife photographer from the period and they will all tell you a changing bag was standard equipment.
Personally, i just don't see Roger being able to. But that's just me.