trying to select a suitable material for the pinhole aperture of a pinhole camera. The pinhole minimum size is limited by diffraction. Is there a change in the quality/quantity of diffraction if the edge around which light diffracts is an electrical conductor or not ? (for ex in old polarizing filters the wires would interact with the electric field of the EM wave)
If the material in which you make the pinhole is uniform on the sub-wavelength scale, and if the material completely absorbs (or reflects) the light that misses the pinhole, the performance of the pinhole will only depend on the shape and size of the pinhole.
OK, now I see what you are asking. You are imagining that some of the light hits the wires and some goes between the wires, and that the polarization filtering occurs only between the wires. That's not the way it works.
Each photon is effectively much wider than the distance between the wires, so every photon that hits the wire grid polarizer "sees" the wires. However, only the photons whose E vector is aligned with the wires can induce current in the wires and be absorbed. For all practical purposes, photons whose E vector is perpendicular to the wires just go through as if the wires were not there. Essentially none of the photons whose E vector is parallel to the wires can get through, even though there seems to be space between the wires.
When a pinhole filter is used, it is typically at the focus of a lens, so a light beam is as small as it can be when it encounters the pinhole. Due to the geometry of the light beam and the lens, the beam waist is usually in the range from 5 to 40 microns in diameter, and the size of the pinhole is chosen to be slightly bigger than the beam waist. The only portions of the beam that hit the material outside the pinhole are portions due to light that has been scattered upstream by, e.g., dirt on the optics. The pinhole removes the scattered light, leaving a "clean" beam. So, light that gets through the pinhole basically never touches the material that the pinhole has been punched into.
As a result, if you make a pinhole in a wire grid polarizer and use the pinhole the way a pinhole is usually used, then most of the light, regardless of polarization, will go through the pinhole. Scattered light, though, will hit the material of the wire grid polarizer. The portion of the scattered light that has its E vector parallel to the wires in the grid will be absorbed. The portion that has its E vector perpendicular to the wires will pass right through the wire grid polarizer.
In effect, a pinhole in a wire grid polarizer will act like an ordinary pinhole for light whose E vector is parallel to the wires, and will act like a sheet of glass for light whose E vector is perpendicular to the wires.