May not be worth the expense but couldn't you buy a universal battery and charge it then keep it in your bag.
http://www.google.com/search?q=asus+...=shop%3A1&aq=f
They have a charger that says it works on a plane but it seems as if it would be slighty more expensive than an extra battery.
http://www.google.com/products/catal...ed=0CEIQ8wIwAg
best of luck