consider there's this Telethon library, for Python3. I've briefly tested if it is at all possible to send messages, spending on the subject not much more than ½h, then playing around a bit. The only issue on N9, according to me, is who's going to write the graphical user interface. for the rest, the whole protocol implementation is neatly handled by import telethon, as usual.
Being the library written in pure python, you can think of doing this also on N900, but then I'm not sure there's any other widgets choice but tcl/tk.