It will all depend on the image sensor and the circuitry in the webcam. Have a look and see what chips are used in there to begin with and see if there is an interface other than the USB you can use; SPI, I2C, serial, RGB etc.
I've got an image sensor and it uses 8bit bus with the image data and a number of lines for synchronisation/frame markers etc.
I would have a look and see what's currently being used and see if that can be adapted and if not you can use a different chip that accepts the same signals.
Or use a chip with USB host and write a driver for it
