I just read through the patch – didn't have a chance to try it out yet – and both the functionality and the implimentation look great!
Can i ask for one smal favor before it gets applied: some test cases showing requests work with and with out the param specified when there are multiple options in the config?
leaving src/test/test-files/solr/conf/solrconfig.xml as is will test the default situation where no writers are lsted ... you can modify src/test/test-files/solr/crazy-path-to-config.xml to include both the default and some new writer and then clone the SampleTest and add some test methods to check that you get the output you expected from various writters.
You don't even need to provide a usefull writer ... even something that just outputs some static text would work, just put it in src/test so people know it's not a "production" writer.
most of the helper methods in AbstractSolrTestCase try to validate that the responses are wellformed XML, but if you want to test a writer that isn't XML, it can still be done using the TerstHarness directly ... BasicFunctionalityTest.testMultipleUpdatesPerAdd shows an example.