The only problem with this method is that it's "push only" - I can only publish with these plugins. No history management, and strictly speaking, not manageable on the same level as using the Metablog API. So, I can't set an account for IBlogExtension blogs - no navigating to the site without manually opening a browser, basically no integration with the current UI. Categories seem to be up to the plugin as well. But I can post. That's one step in the right direction, right?
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