I do not know either much about how the connection would work. I understand the diagram under SITL directory but the ports do not match what the SITL firmware is expecting, so I'm not sure which connection is missing.
I would imagine that the simulator is expected to connect to the firmware and send HIL_* messages using mavlink in SITL. With HITL with X-Plane, I think it is QGC who connects to the autopilot and to X-Plane, and generates the relevant messages. I think that if QGC could be made to connect to the SITL firmware it should work, but as I mentioned I don't even see any options in QGC in the HIL Widget.