Now I have a working RDF Store connector for Semantic MediaWiki, that uses ARC2:s RDF store, rather than SMW:s built-in store. This will allow to take advantage of functionality in ARC2, such as possibility to set up a SPARQL endpoint etc.
The ARC2 connector implements the same amount of the SMWStore API as the JosekiStore, but I'm not yet sure if more needs to be implemented, for the things we want to do (general RDF import/export). Gotta figure that out.
Feel free to try it out, but be warned that it has been only very briefly much tested at all yet!