The RDF-WG's progress on skolomization is welcome complementary progress, but this inability to apply the str() function to a bnode is still a gap in SPARQL that should be addressed. If it cannot be addressed now (due to time constraints) I think it should go on an issues list to be addressed in the next version.
The skolemization work by the RDF Working Group would address your use case because it provides both an external identification of the blank node (considered as an item of RDF abstract syntax) and as a way to use identification found in a query and then used in update operations.
Since a general mechanism is defined by the RDF specifications, this should be used and not rely on a feature unique to SPARQL.
A number of SPARQL implementations do already provide, as extensions, protoype implementations of the skolemization feature. Even if the details differ at this stage, it suggests that take up of this RDF feature will happen.
The timescales of SPARQL-WG and RDF-WG are not aligned and the SPARQL Working Group does not want a dependency on the skolemization work of RDF-WG. Therefore the SPARQL Working Group has decided not to make changes to the str() or iri() functions.
We would be grateful if you would acknowledge that your comment has been answered by sending a reply to this mailing list.
Andy, On behalf of the SPARQL WG