I knew there was something lurking in my old grey matter regarding this.
So, if the police found you actually "asleep" in the car, surely there is the same "chance" as you driving if the keys are in the ignition, in your pocket or even outside the car. Asleep is asleep!
What I am thinking of is a Motor caravan. Forget the laws about sleeping in a car anyway, but a motorhome is interesting. You could even have the engine running for heating. And if the public have access (even on a private caravan park) drink driving I believe is the same.
But as you state, every case is different and for such a common offence there are case laws for and against.