I think the first generation of the C6100 was OEM'd to Supermicro, so this definitely could be based off that design. I have an AMD cluster at work that is the twin-twin setup, I could pull one of those boards and see if it has a usb plug inside (never bothered to look).
I think a lot of us would love to have you figure this out
I have a spare board so would be glad to give it a try but I think there are a few pieces that I are missing that I don't know other than the USB plug.
I assume that the USB data passes back over the FP connector bundle above it? I didn't see any other connections between main board and the expander.