You got it. There's no USB pass-thru on the 9400 base. USB only connects to the remote.

On the 9600, there is a 14 or 15 pin connector between the dock and remote. It is over this connector that the signal for the 2nd High-Speed-Serial connection is passed. Since the 9400 does not have such a connector, the only hope of achieving the same functionality is solely dependent on whether the same 3 PCB traces are available in the remote. (I don't know if they are even available).