Indeed, the ability to use a hybrid car as a backup generator is being talked about a bit, and may come on the market soon. It's not seriously deployed yet. (I even imagined a network of cars connected to the grid which the power company could command to turn on -- if out of doors! -- for spot power at a higher pollution cost.)

For CO2 emissions, there is only really a global total. For other car emissions, it's local, or more to the point downwind. Proper credit markets factor all this in, of course, and let you buy local credits to offset local pollution. But we don't have such markets yet, but we could if we wanted them.

Total cost analysis is indeed what I'm talking about. Of course it gets very hard. If you accept the more dire end of the global warming danger spectrum, you could grind up all the nuclear waste in front of big fans and it wouldn't be anywhere near as devastating.