I would like to edit metadata of the items with URI rather than litterals. For instance, to declare http://fr.dbpedia.org/resource/Centre_de_recherche_bretonne_et_celtique as dc:publisher rather than "CRBC". The only way to do so is to install the Item Relations plugin, to create a new item of type Hyperlink, and then add a "relation" with this item ? Is it possible to retrieve information from dbpedia go display additional information about this item ? For instance, to get the rdfs:label of http://fr.dbpedia.org/resource/Centre_de_recherche_bretonne_et_celtique ?
By the way, "Relation" is more likely a RDF predicate ? Isn't it ?
However it seems that the relations are not added to /items?output=dc-rdf ...
Such things are possible, but it would require a fairly sophisticated plugin that knows how to distinguish a DBpedia URI from a literal in the dc:publisher field and how grab and parse and store the data.
The way Item Relations works, yes, the "Relation" is pretty much the predicate. Its URI could be dug up from what's stored in the database easily.
The dc-rdf output is really just showing the same data that shows up in an item's show page in RDF/XML. Here, too, a plugin could produce a new output that incorporates more of the available data and serialize it differently.