By unmounting the projector lamp and replacing it with a regular photo flash (or any other suitable light source) it is possible to capture the image off the screen with reasonable quality. The film was stopped, not running.
My test is a very preliminary one but I did it to see how it came out just to test the idea.
Since the flash I used was a fixed on the cam unit severe limitation of SLR movement/aim/alignment occoured and the captured frame is only covering about 1/7th of a regular 35mm SLR negative.
The screen was a silver one. I believe a white one would be better for this purpose.
The pic is slightly by light leakage past the projector which hit the screen. The film used was Kodak 200 ASA. Possibly a 100 ASA would be better.
With a separate flash unit synced by wire, optimising the SLR focusing & angle etc. there must be a big potential for improvement.
I am not certain on if the focusing for either the camera or the projector was optimal either, I believe not.
Well, at least it worked.