Figures -- once I posted the question, I came across an answer to another question (http://facebook.stackoverflow.com/questions/8003581/get-photos-from-stream) that gave me what I needed:
photo_comments: 'SELECT object_id, fromid, text from comment where object_id in (SELECT attachment.media.photo.fbid from #posts)'
The example in my answer was a generic description, but turned out to be close to the actual technique (with the photo object missing in the path).
Hope this helps somebody else doing a similarly narrow search.