I have not used the pinhole cap, but I have used the Holga lens for Canon.
It looks like a lens, but it is only a pinhole. I not noticed dust issues with it, but I do not shoot with it often. Although it will mount on both crop and full-frame cameras, the vignetting on full-frame is too extreme. It is best to only shoot a pinhole with a crop camera.
Here is a landscape shot I took with my Holga on my 60D:
Arlington Bridge, Washington, DC by scrappydoggy, on Flickr
Although the shots are in color, I generally prefer to convert to black and white. It's a liberating way to shoot because you do not rely on sharpness or technology. Composition is the critical factor.
Here is an article on shooting with a Holga: http://www.washingtonpost.com/wp-srv/photo/essays/vanRiper/010706.htm
Pros use them too: http://www.jeff-friesen.com/Holga.html
A good resource: http://www.pinholepossibilities.com/artists-who-make-pinhole-photographs.html
Beautiful picture! Thanks for the reading. I only have FF, but my wife has a 600D. I will definitely look at the links you sent.