I know that it won't be thread-safe and I'm aware of the implications of publishing an instance while it's not fully constructed. But I don't see a problem in this case (although I can't rule it out, because i've not tested the smack638 branch).
This is primary because there are no threads involved. And because the connection created listeners will see all fields, minus some parts of the configuration, initialized when they are called from AbstractXMPPConnection's constructor. So what's done typically in a connection creation listener, i.e. adding stanza listeners, iq request handlers, and so on, should work just fine.
If you can sketch a scenario where it breaks, then please share it.