Just posted a patch: Expanded predicates for Relations API that gets one step closer to being able to combine free tagging with RDF. Basically the idea is simple: use the existing free tagging capabilities of Drupal to let users build up a smart domain-specific vocabulary, and use those words as predicates in RDF statements. I’m kind of surprised there didn’t seem to already be an easy way to do this.
The patch above does the step of including the vocabulary terms as predicates for Relations API. I still need to do another patch, probably to taxonomy_xml, to automatically expose them as RDF, and then expose something that makes it easy to output them in nice clean RDFa with the nodes. Once that is done, it seems pretty powerful to me. I like being able to write something and in a structured way say it was inspired by a book, that it implements a philosophy, or that it is useful for some specific goal.
This is partly done at bteaching.com now; the relations are visible to the user along with each node. They aren’t output in RDFa yet, though, that still needs to be done along with the automatic vocabulary exposure as RDF.
Anyway, I wanted to post what I have so far, as its in production on a live site and might be useful for someone else in its current form.