Have you tried opaque material (d 1) with transparent texture? For example white grid on transparent background (holes):
grid-white-transparent
I didn't have time to render better quality (gi "path") image and that's why I added light panels to illuminate the curtain frome the inside (and one light panel is outside the window). Texture size 0.5x0.5cm.
For tiny patterns you should probably increase AA values and/or change filter ("bucket" sampler algorithm) or increase image size ("ipr").
