I agree with Ananke, its not as though he needs the money when there are other pilots needing a job.
I can't see it happening with the royals security side of things. It would make it very easy for a terrorist bunch to cause a crash knowing WilliaM would be on duty wouldn't it, and who is going to protect him... the paramedic who is on his knees dealing with a patient?
Its got to be a non starter before he ever climbs into the red 135 or will a police helicopter be following him to every HEMS job with his protection team?