I recall a story from the New York blackout where a guy used his Prius as a power source: http://www.nbcnewyork.com/news/local/new-jersey-power-gas-prius-hybrid-car-paramus-177998891.html

There's a lot of juice in there, but getting to it is tricky. It's a high voltage, high current system. Firefighters need special training to deal with electric car collisions without being electrocuted. Not a place for amateurs poking around with booster cables.

In the story above, he plugged a 300 watt inverter into the cigarette lighter socket. I doubt you could push much more current through those small wires feeding the socket. I also wonder if the socket stays on when the vehicle is shut down -- not all do. If not, there's a security consideration -- what's to stop somebody from ripping off your "generator?"

Edit: If I owned a Prius, I'd be talking to the techs that maintain it to figure out what the options might be, including small upgrades or modifications with emergency use in mind.