So, I've found these two ‘battery posts’ (at least, for jump-starting the 12V system) under the service panel, in the frunk — strangely, they're on *opposite sides* of the car, nowhere near one another:
However, I need to attach power-leads specifically at the point where the 12V battery...
We use a 50QT ARB 12vDC portable fridge, and have not setup the Model S for it yet. The powered cooler is superb FWIW. No ice needed, and when you get to your destination, you can plug it into AC power. Perfect for beach weeks, etc.
http://store.arbusa.com/Fridges-C11.aspx
It is very thrifty...