So the described casting strategy is interesting but very flawed if your objective is to create a negative cast of the model's face as they normally appear during upright social interactions. Should you want to see a different and usually-considered less flattering facial shape caused by another gravity-altered shape, place a mirror on a horizontal surface and carefully examine your face while facing down to see your reflection. For that reason, the resulting facial cast from lying prone while a cast is taken results in a significantly different shape. Human model faces significantly change shape while lying on their backs as compared to having the top of their head up and their chin down as they normally interact in social circumstances.
