Camera batteries are based on lithium ion cells, which have a voltage of 4.2 volts fully charged, and 3.1 volts safely discharged. You can discharge them further, but they tend to catch fire and blow up.
When you draw current from a li-ion cell, its voltage quickly drops from 4.2 to around 3.7, and holds near there. Then it quickly drops to 3.1. They need a cutoff circuit to keep from dropping further, so you never run a li-ion battery unprotected.
Two li-ion cells in series produces 8.4 volts charged, and 6.2 volts fully discharged, and 7.4 volts nominal. I've tried running the G1X with 9 volts, and sometimes it starts, and sometimes not. I've run the D20 (4.2 volts charged) at 5 volts from a USB battery. That works most of the time, but sometimes it won't start if the USB battery is fully charged.
So the answer to your question is that the G1X might work at 9 volts, but sometimes it doesn't. It doesn't hurt the camera, since it has an over voltage shutoff.
I use an 8.4 volt battery pack designed for bicycle lights. It has rubber straps to attach it to handlebars that attach nicely to tripod legs. It's also very inexpensive!
https://www.amazon.com/gp/product/B00PE6TVCI/
One thing I've discovered about continuous mode is that if the power is suddenly cut off, it loses all the photos in the folder it's writing, up to 2,000 pictures. So I have a test in my script that stops it at 6300mv battery voltage.
I'm not sure the big lithium battery will make it onto an airplane. I've also used a DC to DC converter that changes 12 volts to several different voltages, including 7.4. You can plug it into a lighter plug on a car, or any 12 volt battery.
https://www.amazon.com/gp/product/B000KGKFA4/These links might be useful for wiring everything up:
Use these connectors! The others I bought had very thin wire that broke.
https://www.amazon.com/gp/product/B01IBFNJ3A/Good for adapting the DC converter male plug output to female.
https://www.amazon.com/gp/product/B00J6DM9DI/