I started working on it today, though I won't have as much time the other days of the week... I don't really know which is the best way to go at this moment but I went with building a "libwhatsapp" in C++/Qt. It will then present a Qt class "WhatsApp" with signals for events and some simple public functions (like connect and sendMessage). I think this should be usable in telepathy with possibly some extra Qt code (to interact with the signals) and easily usable in any Qt application.

There is still a lot of work to be done but at least I figured out the complete structure of one message I captured with wireshark and it looks like the current code I have enables me to reproduce that message. Again, I must stress that this achievement was the entire goal for today and that it is nowhere near usable yet.