Is it possible to define a route in the RouteCollection of Asp.net MVC, so that it just does the "URL rewriting" part and ignore the URL generation with Html.Actionlink(...)?
In fact I want to add a keyword between controller and action (controller/..keyword.../action) for certain very special requests. The generated URLs on the pages, however, should remain using the default routes. (controller/action)
Yes you can do what you are asking. You would just create your own route that inherited from the current one and override GetVirtualPath to always return null. This way no Action lookup would be done, but the URL would still function as a routing mapping to your action/controller.
Also by the way, what is happening isn't URL Rewriting, because you using the Routes to define endpoints in to your application. Or in other words an API. So no rewriting is taking place. Think of the relationship between routes and your action/controller as more of a publically defined namespace for the web. Just like a namespace you are defining a specific spot in your application where your action/controller can be found.