I always show this on any plans i do for either Xray or CT equipment. (MRI's are a slightly different animal - I think they usually have a kill switch tied to the door, and the entry door is usually in a controlled location.)
I would ask your local inspector if these are required. I have a feeling that most inspectors would require the warning lights, even if they can't find it somewhere in the code.
I'm really supprised a hospital would not want to install these. Most hospitals are usually more safety minded. Although, they are also very revenue oriented: The first time someone walks into the room and stops a scan that's in progress, I'll bet they will be crying for a warning light.
JAHCO is also something to consider. That's an independent inspection orginaziation that rates hospitals. They inspect both facilities and general operating procedures. If their rules require a warning light, and the hospital gets cited for not having one, someone will be retro fitting one.