[meta]

I wouldn't use the headrest personally. In an accident the magnets will detach and the ipad will become incredibly dangerous to the passengers in the car.

[Retric]

The easy way to look at loose things in your car is would this hurt if it hit me from a 10 story building. However, as this is behind the seat it's probably going to mostly have forward momentum in a crash which means it's far less likely to detach and be tossed around that violently in a crash.

I suspect on average it's about as bad as holding it in your hands.