Upon further reflection, we realized that SharedTestRoutes is not really a hack, but is instead a standin (in the Rails tests) for Rails.application.routes.

* In a real application, action_controller/railties.rb does AC::Base.include(app.routes.url_helpers)
  * ActionController itself does not know about Rails.application, but instead can have named routes for any router
  * SharedTestRoutes are created in abstract_unit to stand in for Rails.application.routes, and is used in
    internal functional tests

actionpack/test/abstract_unit.rb

@@ -205,7 +205,6 @@ module ActionController
     include ActionDispatch::TestProcess
 
     setup do
-      # ROUTES TODO: The router object should come from somewhere sane
       @router = SharedTestRoutes
     end