Changing the front plate wouldn't change the focus, but at the top and bottom of the frame are two little rollers that keep the film flat against the pressure plate. So, if you modded the front plate withoug incorporating the rollers or something else to keep the film flat against the pressure plate there might be a sharpness issue. Swapping a 645 front plate should be feasible.
Not sure what the pressure plate on a 16S back looks like, the one on my 645 back is rectangular.
Or, you could just make super slides, they are very impressive projected, especially when mixed in with some 35mm.