Frankly, not having the publisher->hub payload in the 0.4 spec feels like a failure of a spec, similar to how OAuth 2.0 core is considered a failure. Having a separate spec for the publisher->hub payload also points to a failure, which is exactly what OAuth 2.0 is doing now.
There's nothing wrong with allowing hubs to offer more functionality for publishers, but there absolutely needs to be a common payload. Being able to change hubs by simply changing the hub URL advertised in the link header is critical to a successful standard.
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