app/views/posts/show.html.erb:
<%= my_new_post_path(:foo => 1) %>

config/routes.rb:
ActionController::Routing::Routes.draw do |map|
  map.my_new_post "new", :controller => "posts", :action => "show", :path_prefix => "/a/:a/:b"

  map.connect ':controller/:action/:id'
  map.connect ':controller/:action/:id.:format'
end

test/functional/posts_controller_test.rb:
equire File.dirname(__FILE__) + '/../test_helper'
require 'posts_controller'

class PostsControllerTest < ActionController::TestCase
  def test_show
    get :show, :a => 1, :b => 2
  end
end

$ ruby test/functional/posts_controller_test.rb
Loaded suite test/functional/posts_controller_test
Started
E
Finished in 0.051626 seconds.

  1) Error:
test_show(PostsControllerTest):
ActionView::TemplateError: my_new_post_url failed to generate from {:controller=>"posts", :action=>"show", :foo=>1, :path_prefix=>"/a/:a/:b"}, expected: {:controller=>"posts", :action=>"show", :path_prefix=>"/a/:a/:b"}, diff: {:foo=>1}
    On line #1 of posts/show.html.erb

    1: <%= my_new_post_path(:foo => 1) %>