..and ball joints. The simpler way is just a direct line to qfrc_actuation, taking a wrench to it, as it were. Alternatively you could let the user decide where the force/torque action takes place (a site?), that would allow actual rockets and of course, quadcopters.
Did this go further? Or did you stumble upon something else...? I am looking for a simulator/visualizer for autonomous quadcopter
Yes, this has now been implemented. Actuator transmissions can act on free joints as well as sites. See XML documentation for actuators.