Through the article published by Marco Sagliano whose title is "Open-Source Visualization of Reusable Rockets Motion: Approaching Simulink - FlightGear Co-simulation" is presented how to send, via Simulink to FlightGear, 20 states (signals). These signals can be specified according to the users need creating a xml configuration file (generic protocol). In the article, which is public available, the necessary codes for implementation and explanations are presented.
For the Mathworks team: it might be interesting to add in simulink's Aerospace Blockset library a block that implements what was indicated in the article, at least for me it was very interesting to have a component that allows the freedom for sending signals that can be structured "in the FlightGear side" via generic protocol.