After some googling, I did not find an answer, and turned to the SMW IRC channel. Saruman was helping out and pointed me to the Equivalant URI wiki page. I had looked at that earlier, but now turned to the Import vocabulary page. While this was not what Saruman wanted me to look at, it did turn out a nice workaround. The wiki page shows how to import external ontologies. This trick requires you to craft a wiki page with a special name, starting with MediaWiki:Smw_import_ followed by a namespace. It exemplifies this with the FOAF ontology, which is in fact an ontology I was also using equivalent URIs for.
So, I created a new MediaWiki:Smw_import_foaf page, with this content:
http://xmlns.com/foaf/0.1/| [http://www.foaf-project.org/ Friend Of A Friend] name|Type:String homepage|Type:URL depiction|Type:URL phone|Type:String knows|Type:Page member|Type:Page
And another page, MediaWiki:Smw_import_rdfs, with this:
http://www.w3.org/2000/01/rdf-schema#| [http://www.w3.org/TR/rdf-schema/ RDF Schema] seeAlso|Type:URL
Then, in a Property page, which I want to make equivalent with a property in FOAF or RDF Schema, I can now simply use [[imported from::foaf:homepage]] or [[imported from::rdfs:seeAlso]], in Property:Has_homepage and Property:See_also respectively.
That's a neat trick, /me thinks!