Looks like properties of subclasses have to be references a different way:
@NamedEntityGraph(
name = ChatMessage.GRAPH_FETCH_ALL,
attributeNodes = {@NamedAttributeNode("player"), @NamedAttributeNode("text")}
, subgraphs={@NamedSubgraph(name="MetaData.subgraph", attributeNodes={@NamedAttributeNode("id"), @NamedAttributeNode("creationTimestamp")})}
)