just get an airline adapter
Yes, when you are in a vehicle (car, plane, boat,...) there is power available. For those times when there is no power available you CAN still take a spare battery with you. Just cable the battery to the power port. This is actually better because you can select whatever size/shape spare battery you like. Anything from a case filled with AA size cells to something like this: http://store.cseoffroad.com/d34m.html
I do not see the point of swapping out a batery, you still have the used battery to carry. Why not bring a spare battery and a small cord?
I'm sure some one will sell a flat battery that is the same shape as the bottm on the 17" notebook and you just stack the notebook on top