At this moment we can specify only MapReduceCommand.OutputType and collection name.

public MapReduceCommand(DBCollection inputCollection,
                        String map,
                        String reduce,
                        String outputCollection,
                        MapReduceCommand.OutputType type,
                        DBObject query)
                 throws MongoException

No way for this:

Additional options within out objects are:

• "db" - the db name to output to.

   out : \{replace : "collectionName", db : "otherDB"\}
  

  { sharded : true} - MongoDB 1.9+ If true and combined with an output mode that writes to a collection, the output collection will be sharded using the _id field. See details in the sharding section.