On 3.0.2 I ran an export command on a named graph using a qname for the named graph. That did not work as it did not export any graph, but the full URI for the named graph did work. The qname prefix was defined in namespaces.
C:\Users\jrhyne>stardog data export --named-graph "thx-ng:nm-tr" --format RDF/XML sdtest nm-tr1.rdf
Exported data as RDF/XML to file 'nm-tr1.rdf' in 35.19 ms.
The file nm-tr1.rdf contains only the namespace prefix definitions - including the one used for the named graph IRIs. No error message about named graph not found.
Sorry to say - I am running Windows 7 64