Phil are you talking RED One or Epic/Scarlet? I just realized this forum is RED ONE, all of my response is about Epic, since it looked like thats where topic went. just to be clear.
There is no power coming out of the camera on the SYNC port of the Epic Brain. If the Monstercine switch is not supplying power there is no way this can work. All the other pins on the SYNC port are either inputs or GND. It would be interesting to know which two pins they are shorting when you close the switch though. Other than the switch closure causing a spike which creates the momentary record this type of arrangement will not work for Epic.