I integrated eXist-db with Plone which would give a much better user experience with external systems. Users could manage their content over WebDAV in eXist-db and re-use the same content in Plone. However this is not a complete transparent integration with respect to access management etc. But Reflecto followed a similar approach and it might be suitable for some usecases. I am not promoting eXist-db here since it is a XML database that we are using for a XML project. There are some parts in Plone/Zope that are broken since more than a decade and WebDAV integration is one of these parts.
-aj