You want to include availability metadata expressed using RDF in your TEI header?

This ODD

<schemaSpec ident="testrdf">
    <moduleRef url="rdfxml.rng"/>
    <moduleRef key="header"/>
    <elementSpec ident="availability" mode="change">
      <content>
	<rng:ref name="RDF"/>
      </content>
    </elementSpec>
</schemaSpec>