J.G.
Not sure about wattage and how that all works, but there are a lot of products like this out there. http://www.amazon.com/Wagan-EL2402-SmartAC-Power-Inverter...
It will let you plug in your pump to your cigarette lighter basically. No batteries needed. We keep one in our car at all times. I used it once to pump in the car too.
As for your night out -- I think you have a great idea. Make sure you give yourself extra time to pump. I can't think of any other way to to it aside from bringing another friend with you to find y'all's seats while you stay in the car to pump. Good luck! Have fun.