I'd like to extend Scribus using Python, not just automate things with it...

Currently, that's not really possible. There is work on making it possible to extend Scribus with Python to some extent, especially the GUI. It's now possible to use PyQt to write your own palettes, but you won't be able to use custom Scribus widgets or get into the innards of the application. The Scribus core is unfortunately not well suited to extension from Python. More advanced or tightly integrated extensions are probably better written as C++ plug-ins.