Struan, I apologize for not responding sooner to your excellent post here.
The photolithographic masks you're talking about sound like a reasonable alternative to say Ronchi rulings and the like. I need to send some emails and find out what the manufacturers think of this application.
I wonder if there's not some way to make our own rulings cheaper and easier... Is it possible, for instance, to create line patterns with holography, or some kind of interference pattern? DCG is certainly capable of the resolution, but where does our image source come from?
I do like your idea of getting together with a university though, or perhaps it's possible that someone would be willing to "lend" their rulings for the purpose of contact copying.
I'm very curious to know how you calculated the viewing angle. Pray tell!
On a different note... I'm wondering if one could make a reflective version of diffraction photography. For example, what about etching onto a sheet of metal? Or, I guess, is it possible that these glass versions could be viewed via both transmission and reflection? Actually, when I think about a diffraction grating I recently handled, it was impossible to see the color by looking at the film; it had to be "projected". Hmmm... the physics is beyond me i'm afraid.