I'm not sure if any visitor including myself would be a casual passenger. I ride with Ailsa in her car with a white plate and can prove she's my daughter etc. Never had to do that so I really can't say what judgment an insurance company would make and would probably involve getting lawyer.
When I ride with Linston I always have the chartered sign in the window. Most likely a legal grey area.
I imagine you would have to produce evidence indicating a relationship. The question may be if you are paying for the ride or not.
If a restaurant send a whites plate to pick you up is that a part of the financial arrangement to buy food and drink?
Better safe than sorry.
Perhaps Rob can lend some information.