Is there a way to set 3D parameters (with game data) of an event that has been started via a command instrument? I guess not, but I ask anyway!
Yes, it is possible.
Whenever an event is created by a command instrument, it generates a
FMOD_STUDIO_EVENT_CALLBACK_START_EVENT_COMMAND callback, with the new event instance as an argument. If you listen for this callback and capture the argument, you can use it to address the event instance thereafter.