After setting the proxyusers described in the Ambari Security Guide, I'm prompted to restart several services (HDFS, Yarn, Mapreduce, HBase, Oozie I believe).
However, Hive View will not launch, saying that the necessary proxyuser configurations are not present. HiveServer2 apparently looks at core-site.xml on process start and then refers back to that for the proxyuser check.
Restarting HiveServer2 resolves the issue.
Clearly HiveServer2 is sensitive to proxyuser configuration, so I suggest adding it to the list of components that are prompted for restart in Ambari.