I see during bootup that the usb hub drivers are loading. Is there a place where I could solder a second USB host connection on the 6416? Or is there no hub controller on the board?
You cannot solder an 2nd USB host. You can just add your USB hub and connect to the USB-A.
Actually we did this http://domino.io/product/domino-3x-usb-microsd-combo-tile/ in our Domino boards, adding a USB hub chip. But this could be very difficult on 6416 because of physical things.