The App you use must have XMPP fuctionality. I too have tried this with a few XMPP enabled IM Apps but they all seem quite dodgy. The only one I got to actually wouldn't let me log in to Cryptic's Server.
Beejive on iPhone is so far the only App that works for me for accessing ingame chat channels such as TTS. Though even there it's for one session only, so you have to leave and enter a channel again if you closed the app.