In a recent question it was stated that
You cannot change the focal length of the zone, it is fixed and a characteristic of the physical dimensions of the camera
As far as I know, focal length of a pinhole or zone plate is its distance from the sensor/film; so I'd assume that while going wide is a challenge, attaining focal lengths longer than flange distance should be rather simple, by using extension tubes (or a bellow). I understand that the "lens" would become slower by using same aperture at a larger distance, but the aperture is rather slow from start anyway.
Are there any good reasons to avoid extending pinholes / zone plates further off from image plane?