I’ve considered hanging in the car, but have only tried it while parked. The risk is that you’ll get in an accident, get knocked unconscious, and end with a dead dick. There’s also a risk that you’ll be thrown from the seat, or that your airbag will deploy, any of which event could cause serious damage or worse.
When parked, I’ve tried hanging with a bungee cord extending up over my shoulder and around the upright post of the head rest.
Before I used use anything like this while moving in a car, I’d need to be sure that the hanger wouldn’t cut off circulation. I would not consider using anything other than a vacuum hanger. I’d also need to install some kind of force limiter—something that would cause the cord to separate if force ever approached some unsafe level. I’m thinking of some kind of magnetic coupling that would come apart at 15# or so.
There’s also the privacy issue. What happens if you get stopped by the cops?