If the Model S has all the sensors and actuators for automatic driving, those should be accessible. The cameras might not be on CANbus (it's too slow) but the brakes, steering, powertrain, and radars probably are.
They definitely wouldn't put those on CAN bus, way too unreliable. That's what Flexray was invented for.
I certainly hope those aren't accessible! That's safety-critical hardware, I don't want some overconfident geek messing up their car's driving systems.
Everything is accessible, it's just a question of how many layers of plastic you have to remove to get there. I wouldn't call the bus he found in this article user accessible but he still found it by removing some cover.