That's an interesting scenario I've never really thought about. On a 911 response we have had several times that there are RN's on scene as bystanders (actually had another areas medical director once) where even as a first responder I was deemed in charge of patient care until the medics came, then they were in charge the Nurses and even the doc had absolutely no say.
Also the company that I'm doing my ride-alongs with has done several long distance transfers (few hours one way) where both medics stay in the front of the rig and the hospital sends a team usually an RN or two and possibly an RRT depending on the patients condition. At that point the RN's are in charge of patient care (obviously since both medics are in the front). I'm sure if I was a little more versed in law I would be able to point out the reasoning but for now I'll just accept it.
For anyone else that does transfers with a nurse riding along as the primary patient care provider what happens if the patient codes, does it then become an emergency where you're in charge or is the nurse still in charge?
