I think it may be good idea to add support for "MAM:http://xmpp.org/extensions/xep-0313.html to PubSub. It was already added for Message Archiving component to make it easier to retrieve older messages. MAM can be used with PubSub for same reasons - to retrieve notifications about published items from specified time period, etc. . After we add this implementation we would be able to use same protocol to communicate with both components.