Dario,
yes you can use the IndexReader directly bypassing all of Hibernate Search:

IndexReader locationIndexReader = searchFactory.getIndexReaderAccessor().open( type );
try {
   //use it but read-only!
}
finally {
   searchFactory.getIndexReaderAccessor().close( locationIndexReader );
}

Please let us know if you can find a more efficient way to run your faceting query using the indexreaders, so we can see if we can reproduce a better technique for the benefit of everyone.