you are missing something
The canbus doesnt monitor anything. Some modules, are canbus, such as the center console switch bank (hazards, headlight height, esp, heated seats etc). this is purely connected to the canbus, and when you press a switch, the module sends a request down the canbus, and the ECU turns the required circuit on, or off as required.
The headlights are standard circuits, worked simply by the ECU, just as a standard relay would.
However, the ECU does monitor the loadings, and, if its not happy, it shuts the circuit down. Nothing at all to do with the canbus, which is just a data stream.