Does the USB "back channel" just act as a wireless USB connection between two USB gizmos?
Yes and no, after some tests I think it is like a simple connection but it also functions as a USB hub. You can only use it with the proper USB protocoll - it does not allow to send some random electrical pulses. I think its implemented this way, because you can have several receiver units with several input devices (e.g. keyboards) comunicating with one transmitter device. I have used it with two receivers watching the video on two screens, but I have not tryed to connect more than one USB device (as I won't need it anyway).
So, I really have to tell the system: "Hello, here is a USB host device and that over there is a USB slave device, now set up a connection and send data".
(Thats the only reason I'm using Arduino at all, if it were a simple point-to-point wireless electrical connection, I would have been able to just make an electrical circuit that drives the stepper motor.)