What you might want to do is add new type fields to PEP 345, making it easier to identify and list packages that work as plugins for applications, e.g.
Type: Plugin for MyCoolApp
The MyCoolApp could then use the Type-field to identify all installed plugins, get their installation directories, etc. and work on from there.
Classifiers seem a good way to do that. They’re already defined in accepted PEPs, extensible on demand, used by Web frameworks components/applications/middlewares/things and other projects, and queriable in PyPI. We could use “Environment :: Plugins” and “Framework :: Something” or define a new classifier (not all applications are frameworks, and “plugins” seems a very strange value for “environment”). It would be the first time that a classifier triggers specific processing from distutils though, so we may prefer to define a new Provides-Plugin field for consistency and explicitness. Regards