The RZ is a great camera. The waist level finder has quite good magnification for focusing, but it would be much better if Mamiya made a waist level finder specifically for the size of digital backs.
However one limitation you would have is wide angles. While the RZ has nice wide angles they are designed for 6x7 and with the crop of even a full frame MF digital sensor
You would not get much angle of view with the RZ lenses.
However the RZ lenses are very good even with film it's a very nice camera.
Here is a shot with film:
and here is a crop:
Black and white film could be a good compliment to a p25 RZ combo for when you want a wider angle, plus the option of shooting infrared film
It would also be really nice if mamiya could make a stitch back for the RZ allowing you to shift the back making two captures that cover the full 6x7. Would be very nice for landscape and with a "modest" P25 you would get a 40MP capture.
You can get stitch backs for the Fuji gx680, but they are heavy cameras and the widest lens is 50mm.
However you can get very large files as it can do more than a two frame rectalinear stitch.