It's a completely different computer—compare it to your typical web browser/web server model. The media computer can request display the battery information (the browser can get a list of products) but it cannot hit the accelerator (the browser cannot read other users' personal information). Even if you have root on the media center (your laptop), that doesn't make it so you can run any and all commands on the computer that regulates steering and acceleration (the web server).
there is a gateway between the media center canbus and the critical canbus. the gateway buffers all the data from the critical canbus and rate limits any requests for data which isn't broadcast regularly on the critical canbus.
You would have to get UID=0 on the canbus gateway to make requests 'willy nilly' on the critical canbus. Having UID=0 on the media centre would only help in making willy nilly requests to the gateway.