I am not sure what you'd want from the notification protocol that persisted XMPP doesn't offer already.
http://xmpp.org/extensions/xep-0201.html (last updated Feb. of last year)
Wave is XMPP, with additional Google extensions for a variety of collaboration features. That is possible because the first letter in XMPP doesn't stand for "XML", it's the (annoying but common) "eXtensible", suggesting that domain-specific additions are not only accepted, they are encouraged.
XMPP might be the answer, or maybe dialects on top of XMPP. (haven't looked too close to see what's possible).