According to the devs, mostly anything should be possible if you have the required coding knowledge, which is in Python. The GUI should be modable with desktopX but I remember reading somewhere that they (devs) needed to release the API for desktopX (forgot what these letters mean -.-; ) in order to modify the current UI. I attempted to change some GUI's by switching the pictures for my own using desktopX, and it didn't do anything.
It looks to me like everything you are asking for should probably be doable, but it won't be as easy as modifying XML. Of course I'm no pro in this so I could most definetly be wrong. But yeah just like you I'm interested in modifying some stats, and adding some new screens to the game.