Have you ever tried getting live video stream from external camera connected to Android device to later process it with OpenCV functions?
I've found some libraries that seem to allow external camera connection and video grabbing, ex. UVCCamera, but OpenCV supports only device's rear and front cameras. Maybe there's some other computer vision library for Android that will do the trick? Or some changes to the device (like rooting or sth)? I'll be glad for any clues!