The broadcast code was just recently ported form the old Enterprise version of ICEfaces which no longer exists. It hasn't been as ruggedly tested as we'd like (think of it as an early access API) so that's why it's not as fully documented yet.
There is a blurb in the release notes for 1.7.1 regarding "New Broadcast Render capabilities". The broadcast flag on the RenderManager or SessionRenderer is one part but there is also a context param that looks like it needs to be set to true:
From the documentation and the source code I infer that this needs to be true no matter what you do programmatically.
The other thing you probably need to do that doesn't appear to be documented is to create a separate JMS topic for broadcast rendering. It follows the same steps as creating topics for AHS but has a different topic name. The AHS topics are:
public static final String CONTEXT_EVENT_TOPIC_NAME =
public static final String RESPONSE_TOPIC_NAME =
The broadcast rendering topic is:
public static final String RENDER_TOPIC_NAME =
As for combining the portlets into a single war, you've already noted the benefits if, as you've said, you have certain kinds of inter-dependencies between the portlets. Usually if you are doing IPC