What I would like to see here is the addition of credentials directly to the httpClient, such as:
1. Make httpClient's type to DefaultHttpClient
2. Use httpClient.setCredentialProvider()

See: 
http://stackoverflow.com/questions/7725043/how-to-make-this-rest-get-method-in-java and
http://hc.apache.org/httpcomponents-client-ga/tutorial/html/authentication.html

The idea is that you can set the credentials on the httpClient, then the RdfRetriever and NamedFieldsRetriever will not need to be modified.

p.s. There may be value in also including a StandardHttpRequestRetryHandler:
https://github.com/futures/benchtool/blob/master/src/main/java/org/fcrepo/bench/BenchTool.java#L122