Amos, I'm aware that I used the word "illusion" without qualification. Maybe better would be to say that if the medical profession are aware the hallucination experienced is a well-known symptom of what the patient has experienced, and there is a good physical explanation for it, why should they pretend that it's anything other than a hallucination? Sure, you don't raise the point when the patient is still unwell and doesn't need any self-doubt. But if they bring it up when they're well again, why not? (I know in this case the physical explanation still needs some work, but in general.)
I guess I'm wondering why this particular hallucination should be treated in a different way to vision distortion, bright lights and strange sounds during concussion, for example? Or again, the hallucinations in schizophrenia where a chemical imbalance causes people to genuinely believe that God is telling them to do things.
For a personal example, earlier this year I had a fantastic roller-coaster ride for I guess about 5-10 minutes - whilst sitting flat still in a sauna, very hot and a bit dehydrated. Did I really swirl round the room like Neo in the Matrix? Did the room really lift off the ground and swing round the Earth several miles up? It sure as hell felt like it, but somehow I doubt it. :-)
Yes, those cases presuppose it's a hallucination. If it's not a hallucination, for an OOB the key point is establishing that the person wasn't merely extrapolating the view from their situation, but could actually see things they couldn't otherwise have known. There have been a few attempts at this with mixed results AFAIK - certainly I've heard of nothing conclusive here. I'm not so worried about McGrath's "how does it work?" question so much as "does it work at all?".