@copygirl @Dirk yes, I also get the feeling this would not work in a compliant setup but it seems like a good idea to test this in e.g. a federation test suite.
Maybe @evanprodromou would know how this should work, or would know of someone who might be testing this kind of scenario.
@CrazyDuck @confluence ahoy!
No, I don't think so? As far as I can tell all extensions were public, in particular Jingle: https://en.wikipedia.org/wiki/XMPP, https://en.wikipedia.org/wiki/Jingle\_(protocol)
Disclosure: I worked on gTalk towards the end of its lifetime and was the person responsible for (sadly) turning down federation.