In view of the stupid questions that I've been asking, I thought I'd better provide and update, should anybody care.
Things are measured by various network nodes in a networked car so the instrument cluster no longer has wires to be able to do it directly. The Instrument Auxiliary (IA) node delivers the expected signals and much more to the instrument cluster.
Instrument cluster information is enhanced by employing additional LEDs in the cluster, and by loading extra functions onto existing indicators; such as blinking to indicate a fault with a lamp.
The measured quantities and signals have been identified.
The IA node is also registered with the immobilizer module; with a trust relationship enforced using SHA. When the IA node notices that the car is being driven despite the immobilizer telling other nodes to prevent that; or if the immobilizer module cannot be trusted, the IA node will be creative in how it manipulates the information displayed to the driver through the instrument cluster and the brake warning light.