Thanks, I'd seen that manual but it lacks specific information about the accessory jack. I've since communicated with Clive Tobin (many thanks to Clive). Here is what he said, in case anyone needs the information in the future:
"The Accessory jack is intended for running a Bolex torque motor that does not draw too much current, or a relay for turning on lights. The output is something like 8 volts DC which may suffice to trigger a still camera. I don't think I have any information on what voltage or connection would be needed to trigger the still camera. If you need a contact closure instead of a voltage you could use a little low-current relay for this."