The requirement to include components such as spaces and square brackets in the GET URL for the server is severely problematic, because the eXist httpclient:get function rejects them; if they're escaped using percent escapes, the Solr server doesn't like them, but if they're escaped using XML entities, eXist rejects them.

This may be gotten around by using httpclient:post, with httpclient:field elements; see the testSolr.xql file currently in the Graves app, where I've been doing some early testing.

I may actually need to go back to running my own local Solr and local eXist for this dev work, because it's too awkward having to edit stuff in the live site (which is the only way to call the real Solr instance).