The cable release kit cost £13.75 off ebay. It has a square housing that fits neatly around the lens, cut away to give access to all the holga bits. The rectangle bit (top left) has a sprung metal rod that operates the shutter release and it's threaded for a standard shutter release cable (one came with the kit).
In practice it seems to operate as well as a shutter release cable on any other camera, the action is a little bit 'firmer' though.
Time will tell how robust the system is
More on the mod
in my blog