My concern would be fixing the camera and frame to ensure that the film was square on to the sensor. With a conventional slide copier for 35mm film used on an SLR, this is addressed either by having the film slotted into the end of a tube which contains the lens and is attached to the camera, or by having a film holder attached to the front of bellows that have a macro lens attached.
I can see problems (down to my own inherent clumsiness, no doubt) in getting a free standing film holder to stay in the correct position and accurately parallel (at least within the limits of depth of field when the lens isn't much stopped down - stopping down reduces resolution and with a flat subject brings no advantage with the increased depth of field).
I think I agree with Stephen that something like this needs a device to hold the camera in the right place and sensor parallel to the film. I have used a handheld camera to copy a print, and it was very difficult to get the camera positioned right, in fact I'm pretty certain I ended up with keystoning.
Something like the base of a Chroma, with a sliding unit combining a base for the camera to sit on and the frame of a square filter system might work. Then you screw a lens adapter onto the front of the lens, clip the filter frame on, and You should be able to get everything nicely stable and parallel.