The point about "locking" your head position is valid. It'd be too rigid to play.
However, I think Polyphony may be able to adjust the "free play zone" such that head tracking doesn't kick in until you turn your head sufficiently. The other way is to trigger a button to enable head tracking.
PD would have wasted their backseat modeling work if we cannot turn and look behind in premium cars.
There is a button for looking back on the controller, so it's not needed for that - I would probably throw up my dinner if they implemented it that way. Looking back is d-pad down on the controller or typically one of the R buttons. Almost everything in this game for any type of supported controller is configurable though, so I'm sure we'll get plenty of options to choose whatever we prefer.