Wow. Anything else I should know about that is verboten? I mean come on.
1. Multiple exposures are easily made with M-bodies using at least two different methods, one of which has been referenced above.
2. Jay Maisel made incredible images of Stonehenge at night with an immense full moon in the background. One of the few times that he "manipulated" an image, shooting the moon with a rather long lens and then shooting Stonehenge with something rather less long - on the same frame. Yes, he used a Nikon SLR, but that's beside the point.
God forbid we should ever underexpose, overexpose, shoot "too" wide, shoot "too" long, shoot multiple exposures on one frame, "Hamilton-ize" lenses, physically manipulate emulsions, embrace noise, grain, or do anything else with a medium that isn't approved... But, I understand... It is hard to come up with valid lens test data when shooting double exposures.
Amazing.