My research since starting this topic about two years ago shows:
The easiest solution (but not the cheapest) is to buy a Copal #0 shutter and mount a Skink Pinhole Pancake Pro Kit for Copal, Compur, Prontor #0 shutters in it. You would then mount that assembly in a lens board for a technical or view camera and adjust the distance from the lens to the sensor to suit the focal length that matches the size of the pinhole you intend to use. There are tables which show what size pinhole is the optimum for a given focal length—see below.
You then control the exposure from the flash synch socket on the shutter as usual.
A less expensive option is to glue a pinhole holder from Skink and others onto a lens board and to control the exposure using the shutter and flash sync on an external shutter or lens which you could hold in your hand. Alternatively, you could use Doug Peterson's suggestion of holding something metallic against the sync cable going to the DB. See Doug's post above.
Kapture sell a sensor activation device for Phase One backs and Leaf have a cable with a switch. see the discussion at: http://forum.phaseone.com/En/viewtopic.php?f=45&t=17803&p=84856&sid=d85825128a88917cd84bc0f2b9e128e9#p84856
As Doug says, you must cover the pinhole so the sensor is in complete darkness before ending the exposure by removing from the sync cable the metallic something used to fire the sensor.
You need to do this because, when the sensor is reading out data/doing black calibration, there must be complete darkness otherwise you will get strange image errors.
I originally wanted to use my Hasselblad 5xx body for pinhole work but I have abandoned that as you would have a fixed focal length of about 73mm, which is too limiting in my opinion. In theory, using a view/technical camera gives you the flexibility of different focal lengths but I have yet to test it.
For every focal length there is an optimum pinhole diameter. Lord Rayleigh's formula in Excel is =1.9*SQRT(0.00055*x)*1000 where x is the focal length and 0.00055 is the wavelength of green light in mm.
The above comments also apply to using zone plates and zone sieves with DBs.
I have ordered various components and I will edit the above to reflect reality!