Yes, I'm on 3.4.4. But 4.0 is similar in the file your found (Spatial was renamed to Node3D in Godot 4.0, but works almost the same). I know I could get it working with a recompile, but I don't wish to do that as it will only work for me. Might still be worth trying just to see how it feels, but the idea was to release it as a plugin to help other people.
That said, I still have full control over user created Nodes. Meaning the Space Navigator could be used to position/orient objects, or move user cameras. This is less common, but still could be useful. I already tried it, I can get the selected object like this:
var editor = get_editor_interface()
var selection = editor.get_selection()
And then do whatever I want. However, I really wanted to use it to fly around the scene.
One other option is to spoof input, which would get passed to the editor. I know I can trigger fake mouse/keyboard events, but this will greatly limit the kind of 6DOF controls I want. However, it could be a compromise.