Jeni's Musings

I completely agree with Richard here - best to represent it purely in RDF. Hence, I have a preference for your second example, with:

@prefix event: <http://purl.org/NET/c4dm/event.owl#>.
partnerships:Membership rdfs:subClassOf event:Event. 
partnerships:hasMember rdfs:subPropertyOf event:agent.
partnerships:hasPartnership rdfs:subPropertyOf event:agent.

For completeness, another way of representing that information (similar to the Named Graph approach) would be in N3 (a superset of RDF), and using a graph literal.