It's not actually necessary for the namespace prefix to be slash. You can use any valid XML namespace name - say,
foo - as long as you specify xmlns:foo="http://purl.org/rss/1.0/modules/slash/". Some aggregators may not understand if you do, though - it depends how well the implementer understood XML Namespaces.
C9 also supports the Comment API, at least for the feed for the first message in a thread. It's not supported for the individual post feeds at the moment.
In browsing through the feeds I realised that C9 actually does associate posts with a parent. It just doesn't present a fully threaded view (e.g. CodeProject.com's forums).