It is recommended not to charge Li-Ion batteries with [square wave] power inverters. Why can’t you use a 750 to 1000ma USB adapter for a cigarette lighter with a Micro B USB cable?
Charging of the Jetpack is only supported by using the provided charger from VZW.
Anything that you do beyond that to charge your device is on your own and a risk of voiding your warranty. Go ahead and use your fancy charger if you want, but don't come crying back to us when VZW wont replace it for free after you cook the battery or Jetpack in the process. My suggestion is to proceed with caution.