This looks like a very interesting option http://www.godox.com/EN/Products_Portable_Power_Inverter_LP450&LP800x.html
It can be charged via the mains or a car battery. You can carry spare battery's that clip on/off and it looks like it could recharge the OW capably a few times? Downside is cost, but if it significantly extends the range of the OW then it's worth it!
The inverter and battery together are about half the weight of the OW (just over 6kg) so should be relatively easy to carry in the supplied shoulder bag, a backpack, or possibly even in one of compartments of the One Penny Bag by @Jimmers75 (I have not checked the dimensions).
It is pure sine wave and 900W or 1400W. Can anyone see a reason why this would not work?