Just to add a general opinion in here:
Having worked with Setuptools' entry points, and a little with some Zope pluginish systems (Products.*, which I don't think anyone liked much, and some ways ZCML is used is pluginish), I'm not very excited about these. The plugin system that causes the least confusion and yet seems to accomplish everything it needs is just listing objects in configuration -- nothing gets activated implicitly with installation, and names are Python package/object names without indirection. The only thing I'd want to add is the ability to also point to files, as a common use for plugins is adding ad hoc functionality to an application, and the overhead of package creation isn't always called for. hg for example seems both simple and general enough, and it doesn't use anything fancy.
Purely for the purpose of discovery and documentation it might be helpful to have APIs, then some tool could show available plugins (especially if PyPI had a query interface for this), or at least installed plugins, with the necessary code to invoke them.
*Maybe* it would make sense to generalize the discovery of plugin types, so that you can simply refer to an object and the application can determine what kind of plugin it is. But having described this, it actually doesn't seem like a useful thing to generalize.