As I understand it, the two 12V power outlets are turned off when the Prius is shut down, so they can't be used to provide power to turn the beast back on. You are back to a battery jump, I fear.
The comment by "Mr. G" is incomplete. The 12V battery continues power to the SmartKey sensor system and the remote access system when SmartKey is not installed. There are several sensors, I believe, and they are enough drain on the 12V battery to make the car go dead after a couple of weeks of being unused. Other things in the car still have power, such as parking and brake lights, internal lighting, and even (I would bet) the clock in the radio, and if enough of those things were left on, there might be an even greater drain on the obviously inadequate 12V battery.