In general though i wondering if steping back a bit and modifying your
request handler to use a SolrDocumentList where you've already
flattened
the ExternalFileField into each SolrDocument would be an easier
approach
-- then you wouldnt' need to modify the ResponseWriter at all.
Consider using a search component at the end of the chain that adds
fields to your document... this way things work for any writer (json,
xml, whatever)
We really should add an example to do this... but in the meantime, a
good example (though a bit complex) is with the local lucene:
http://sourceforge.net/projects/locallucene/
this adds a calculated distance to each document before it gets passed
to the writer
