The Photo’s give you the general Idea of the mechanism. I wonder How big dust is(What a question) would it seep into the pinhole.
Hmmm THe pinhole is, well the one they specify has a diameter of .03mm or 300µm(micrometers)
the Pixel Pitch of the new Canon EOS 5d Mark II is 6.4µm square
So It would seem necessary to make sure the dust is larger that 300 micromete4rs or much much smaller than 6.4 micrometers or else your just screwed.
So how big is dust Well according to the CRC it ranges thusly
Particle Name Size Micrometers
Coal Dust 1-100
Cement Dust 3-100
Insecticides Dusts 0.4-10
Atmospheric Dust 0.001-20
So no one should never use this i suppose unless they were going to clean the sensor which is probably not a good Idea anyway