Documentation on
http://getahead.org/dwr/server/servlet/plugin lists
"org.directwebremoting.WebContextFactory.WebContextBuilder" and "org.directwebremoting.ServerContextFactory.ServerContextBuilder" as two valid extension points that can be used in XML.
This is however wrong, it took me quite a while that the correct strings was:
"org.directwebremoting.WebContextFactory$WebContextBuilder" and
"org.directwebremoting.ServerContextFactory$ServerContextBuilder".
This might be the case for some of the other entries as well, but I found at least this while trying to plug-in my own ServerContext.