I'm going to be doing a similar set up but with around 10 or so Holgas (cheapest camera in 120 I can get in bulk) - The shutter spring has a fair amount of resistance and the holga's aren't known for mechanical (or optical) similarity camera to camera - so to get them all to fire at the same instant would rather fast acting and powerful solenoid (under load) - not to mention I would need 10 of them ...

For now I'm going to work with them all open in the dark perhaps with some slow film and ND gel in front and use a few slaved strobes ...

If I get round to testing a few actuators I'll get back (;

have you thought more about it ? the Holga uses the same 'press' style self cocking system, not sure whats inside your copals tho