Thank you for your quick and detailed answer.
Actually, what I was trying to do is a high-resolution capture of the scene. I know there is already such a feature in the Unreal editor, but since we are targeting rather small PC builds, we would like to spare VRAM and therefore only do “native resolution” captures to reconstruct a high-resolution one, using some sort of stitching.
May I ask you if you know another way of doing this (maybe without changing the projection matrices) ? We would really like to avoid modifying the Unreal sources, if possible.