One solution is to take an ap out of radio preference for the purpose of connecting the chromecast initially in a public place and request users to go there to initially have their chromecast connect. Then the chromecast will work after that.
Another solution is to have people with a chromecast come into IT, have their chromecast connect to the network with an ap that is not in a radio preference group there in the IT office, then let them go back out into the environment since oddly it is only upon the initial connection that there is a problem.
Another solution would be on a per case basis, to lookup the ap the client is currently connected to from the controller reports ( perhaps a dorm room for example ), remove that ap temporarily from the radio preference group, have the chromecast connect, then add it back to the radio preference group.
The behavior could change with the next hardware version of chromecast that comes out, or after some new firmware update on the chromecasts.