I had some trouble getting it working too, so I ended up hacking the plugin source in a couple places (grabbing the $params and using call_user_function_array).
Take a look at my slightly-forked version in https://gist.github.com/755930
As for calling the API, my Python XML-RPC client calls callWpMethod from where it seems to be exported, in the “wpext” namespace (like “wp.” or “metaWeblog.”), so in Python that’s something like server.wpext.callWpMethod(user, password, ‘get_post_custom’, [5067]).
Hope this helps!
