I have been thinking softness in pinhole photography will be greatly affected by diffraction limits of the sensor. I have posted a test somewhere that shows this effect and my conclusion was f13 is the D2X limit before the effect starts to show. So a pinhole giving an aperture of around f200 is really challenging that, and cannot expect too much in the way of sharpness at all.
I have just found this link which looks interesting.
http://galactinus.net/vilva/pinhole/index.html
If you scroll down to ' The Retro Way : My Other Photography ' then this guy has tried out more alternatives than I have.
Don