This is a very good idea. I actually thought about it since the cold season began last year (2013). I bought a cheap 12v heater from Harbor Freight and powered it with my portable 18 ah 12v jumpstart battery. It actually worked quite well as a space heater when used very close to my body during fall. During one test it ran for more than two hours. I assume the load was small for the battery. It can defrost only a small circle on the windshield since it's airflow from the tiny fan is very low. It's heat is insufficient in frigid winter. This was just an experiment.
My real goal (which I haven't tried yet) is to use my 120v Vornado room circulation heater which comes with it's own thermostat, powered by my huge 55 Ah Wagan 2546 Solar e Power Cube 1500. It comes with 2 AC outlets, inbuilt-circuit breakers, 1500w (3600w Peak) inverter...more specs in the Wagan link below.
12v Auto Space Heater:http://www.harborfreight.com/12-volt-auto-heater-defroster-with-light-60525.html
12v Jumpstarter Battery with cigerrete lighter socket:http://www.powerstationusa.com/brands/Powerstation/product.php?mod=PSX3
Wagan 2546 Solar e Power Cube 1500:http://www.wagan.com/index.php/products/solar-innovations/solar-industrial/2546-solar-e-power-cube-1500.html
Vornado Cool Touch 750/1500W VH2:http://www.vornado.com/heaters/VH2-Heater
With the limited range that current 24 Kwh LEAFs offer, I think any type of heating that doesn't use energy from the battery pack is helpful. I almost never used my 2012's (non-heat-pump) heater during this very cold winter this year because the range had already dropped a lot due to the cold. The WAGAN Solar e-Power Cube is very heavy at about 100 pounds but its still less than an average adult's weight and the LEAF is designed to carry 4-5 adults. So the battery's weight shouldn't be a problem. The Vornado heats a small room within a few minutes and it circulates air very quickly. So heating a car's cabin shouldn't take long.