this is somthing i have thought about, one idea i had was to put in a second lighter next to the first one.
the second one would only come on with the ignition.
so if your phone is plugged in hidden it will still charge on number one.
but your camera will turn off, when you leave the car on number two.
one thing is worth thinking about is, if you where to have a minor bump the first thing you would do is take out your keys when you get out of the car.
but you still need the camera recording. this wont happen,
the camera connected to the acc supply will turn off after a couple of minutes.
what camera have you got? and what sized card are you recording on too?
just reread your first post and you all ready have two sockets so you only need to change the supply for one, ferretflooozy's idea sounds good to me.
but please do let us know what camera you have and maybe a link on where to get one. :)