There might be some mods for hot wiring the 12V plug - you might search for that and use a 12V USB converter.
Some cameras have internal batteries. I have the Roav ProCam C2 and it has a back up battery and will trigger if there is an impact.
If you don't find what you need you may want to look at a fuse tap like this. You will need a circuit that is always live. I have never used myself but they seem to be popular. Also the fuse box cover may not fit at that point.
Amazon.com: Uriveusa 12V Car Add-A-Circuit Fuse TAP Adapter Fuse Holder with 10A, 20A Fuse [Upgraded Fuses] - (2 Pack) (Mini Fuse): Automotive
This will get you 12V. Aou will also need a 12V to 5V USB port. There are lots of options there.