Nick,
We were confronted with this very issue in the US EMS business....vender operated programs used single pilot, medical crew member sitting in the "co-pilot" seat on BO, BK, and B-412's. They operated radios, assisted in navigation, tuning aircraft radios, and other "crew" tasks. Because they were hospital employees and not employees of the Part 135 operator.....then they had to be considered passengers and not crew. Each flight therefore had to be operated under 135 whereas in a non-patient , crew only, hospital operated service (where pilots, mechanics,and medical crew were all hospital employees) the flight could be operated under Part 91, until the patient was onboard.
It challenges the reality of the situation but the legal aspects are tremendous when this issue arises. As much as we wanted to consider the medical personnel as crew....the law clearly prohibited that in the legal structure that was being used at that time.
I guess the medical crew could have been contracted to the helicotper vender....and then the whole crew and aircraft could have been further leased back to the hospital.....as a way to get around that but then other legal issues arise.