>Previously, I read the SparqlExtension documentation and came to the
>conclusion that I would have to mirror the entire content of the
>endpoint in a local triple store before being able to query it in my
>wiki. This put me off using the SparqlExtension, because I only need a
>tiny fraction of the data in the endpoint (probably less than 0.1%) in
>my wiki.

The only thing that gets mirrored is the RDF data from your own wiki - included in the extension is a script that dumps all the wiki RDF data into the triple store.  If you don't run this, the triple store will just get progressively populated whenever you edit pages.