- what's the easiest way to have a PC act as a USB perhipheral device, to allow it to be connected
  to another PC's host-controller?

    (yes, that sounds bizarre, but it would allow the first PC to:
            - act completely as a keyboard (allowing it to send any keystrokes), or a mass-storage
              device, or anything

            - have a fully software-based KVM (sans V)
                    (ie. be able to receive a USB packet in on a normal host-controller interface,
                         and be able to decide to route it to an "outgoing" USB connection, rather
                         than interpret it internally)