If your Handler can write out data only using a specific writer, you have the flexibility of overriding the 'wt' in the handler. Register your own writer in solrconfig.xml.
(a handler can even go so far as to "fail" in the inform(SolrCore) method if the writer it expects is not present)
The ShowFileRequestHandler and RawResponseWriter are good examples of this model (although it would probably make sense to change RawResponseWriter to implement BinaryQueryResponseWriter at some point)
It is incongruous to have SolrQueryRequest.getContentStreams() but nothing similar for SolrQueryResponse.
Only if you are use to thinking of things in terms of the servlet API : )
generally speaking, the majority of Request Handlers shouldn't be dealing with raw character or binary streams ... they should be dealing with simple objects and deferring rendering of those objects to the QueryResponseWriter to decide how to render them based on the wishes of the client ... there are exceptions to every rule however, hence the approach described here where the Handler "forces" a particular response writer.