I agree with Jim on this. I fear the 35mm film in the middle of the opening will curl on the sides unless it is properly supported.
Of course, this might add to the cachet
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
Film flatness and focusing accuracy has been a concern for me, which had kept me from doing it for years. Until one day when I picked up a used bulk film loader from a camera swap meet with some film left in it. I opened the loader and took the remaining film out. The remaining film was in a unknown condition anyway so I wasted it. I found something about the unused film. The film was naturally curled or rolled in the direction of the the film's length. But I found that it was completely flat on the vertical direction. This was not what I usually saw on a processed film that curls in both directions. This prompted me to cut a stripe of it and put it in the camera and closed the back. Then I had the shutter fired in B to look at the film from the front side through the opened shutter. The film looked very flat to me. I could not imagine that there would be a focusing problem. Of course I can say that all 35 mm long roll films will be like that. I will need to investigate it.
The reason I want to shoot panorama with 35 mm films is because I have a lot of 35 mm 100 ft long roll films. I will never finish shooting them through my 35 mm cameras. I will shoot 46 mm films on my Pentax 67-ii too. I have quite some 46 mm films left from a student portrait job with a Camerz camera. I have some 70 mm partial rolls too. That I can't use in my P67-ii though.