Supported or not :-/ I may have found a way - here's what I did:

 1. created a Force.com Site,

 2. saved a **Session.page** containing the following:

        <apex:page contentType="text/plain">{!GETSESSIONID()}</apex:page>

 3. edited the list of Site Visualforce Pages on the Site to include it,

 4. viewed the page at the Secure Web Address (mysite.secure.force.com/Session)

![session id][1]

This (apparently) yields the Site Guest User's current Session ID. Taking the string and placing it in the `Authorization: OAuth ...` header works when subscribing to a topic at `/cometd/27.0`.

**Edit:** more testing needed...

  [1]: https://i.sstatic.net/3fEga.png