I wonder if it is possible to do impressioning with some sort of simple key blank. My guess is no. A standard key blank has full bitting size on both sides. My guess the key gets stuck and shows impressions either on both sides or always on the same side but gives no indication where to remove material. A halve key blank, i.e. one where the rear bitting has been removed, will probably give good indications, but non on all levers because due to missing springs friction between the levers will "overlift" some levers and the process gets stuck.
What do you think?