If the power plug to the camera is USB it will be 5V, and there will be a 12 -> 5 V converter either in the plug you push into the dash lighter, or some have it in a little box a foot or 2 up the wire from the 12 V plug.
So you can use a multi channel USB charger to power the camera instead of the provided one, just have to be sure it is a good quality one and it can give at least 2.0 A on the port for the camera.
This way you have other USB plugs ready for charging other stuff in the car.
Also if you buy USB wires to bridge the gab between USB charger and camera on windscreen, make sure it is quality cobber USB cables and not cheap CCA ( Cobber Clad Aluminum ) cables with too much resistance in.
I personally missede getting rear ended as the cabel from the front of my car to the rear camera was a too long and too cheap one.
Check the specifications for the USB port in the car. Many of them do not have sufficient output to power a dash cam. In my experience only 2 of 4 vehicles with USB ports had adequate output for my camera(s), for the other 2 vehicles I had to use a 12V to 5V USB adapter in the accessory (lighter) socket.
I see what you want to do. However using the 12v car adapter or a hardwire kit would be the way to connect your camera. Keep the lead out of the way by tucking it in to the weather seal and headlining.