Can the USB port on the Tau2 or Quark2 VPC accessory be used to just supply power, and use the coax connection for analog image?

Yes, the USB port on the Video / Power / Communications (VPC) accessory provides power, and nominally draws 180mA, 500mA at startup. A standard USB charging device can also be used to power the Tau2 and Quark2 cameras with a USB-A to USB-mini B cable. The USB port also enables camera communication via the Camera Control GUI, but the camera will operate autonomously without requiring a PC or remote camera control. The MCX coaxial connector on the VPC provides the analog video signal in either NTSC or PAL format. USB and video cables are provided with the VPC accessory.