As far as I understand the wave-optics effects,such e.g.diffraction at a caustic, are hardly observable in the gravitational lensing. Are there any observations where the diffraction paterns of the visible light have been seen? If not visible, which wavelength?
I'm not sure how to define the "edges" or obscurations in a case of gravitational lensing. The image we produce on Earth is the combination of our local telescope lens elements and the "grav lens" a zillion Kessel Runs away from us. Even if you use the premise that the black hole itself is a central obscuration which should cause a Spot of Arago, I don't think any other diffraction-like effect is observable from such a great distance. All the wavefronts of light which passes thru the GravLens region continue to expand, ending up near-parallel from Earth's viewpoint.