Headline messages

I am trying to push a headline message to a chat that's I am participating in.  In browsing the docs I assumes chat.sendMessage(Message m) would allow me to set the Message.Type of the message.  However, I am seeing that the message type gets overridden.

The comments imply that the message type will be consistent across the chat session. 

If that's so how can I inject a headline message?

-Tanya