I had the same concern. I currently have most of my accessories running of a suitable multiport usb charger on the rear 12v, including my dash cam.
It reliably records off external power for 10 minutes after locking the car or last opening the door, ect. As long as you're not looking for true parking mode capability I think just using the 12v adapter is a great solution. It won't have an real impact on the car's electric system and will at least get a little footage after you've left.