Would it be a good thing to have a JDBC driver to access Solr? This would help people only knowing SQL syntax. And you can use things like Hibernate on top of that. Maybe this is a good alternative to Solrj. In the first run I would not allow any joins.
I think of a SQL syntax like
SELECT * FROM SOLR WHERE name='Albert' OR name='Einstein'
which gets translated to a Solr query like
q=name:Albert OR name:Einstein&fl=*
What do you think?