In OpenDocument v1.2 (see https://docs.oasis-open.org/office/v1.2/OpenDocument-v1.2.pdf) section 4.1 describes how documents may store arbitrary metadata as embedded RDF graphs. Libreoffice appears to contain API calls that allow the caller to maintain this information. However, the policy (in Libreoffice) seems to be that support for showing and editing the RDF metadata (ODF v1.2) is delegated to plugins/extensions. There seems to be no core (i.e., non-plugin/extension) support for RDF metadata, not even for showing an RDF metadata that already exists in a document. Given that RDF metadata is such a central and well-documented concept within OpenDocument specification, I think it makes sense for Libreoffice to provide at least some support for RDF metadata.
This request sounds pretty much like bug 30709 - in general it would be indeed nice to have that. *** This bug has been marked as a duplicate of bug 30709 ***