Why not just make a generic XMPP/Jabber client first and then extend it to support TLS. So you don't bite off so much at once.

Dunno why you would have to integrate anything with MacTCP... TLS wouldn't have anthing to do with the transport later other than sitting on top of it? Or perhaps I am missing something.

I'm thinking (HTTP -> TLS) -> TCP or does TLS require closer integration with TCP than I am thinking?

If you make your app GPL you may be able to just use MatrixSSL as its quite small. I'm not a GPL propoent per say I'm acutally lean MIT/BSD... but its there and might save you headache.