Is Diffraction of light observed in gravitational lensing/microlensing?
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.