Preserve Case in Route Parameters with LowercaseUrls enabled

I am using routes.LowercaseUrls = true; in my MVC 4 application which is working great. However, parameters will also get lowercased, so if I have a route like

routes.MapRoute(
    name: "MyController",
    url: "foo/{hash}/{action}",
    defaults: new { controller = "MyController", action = "Details" }
);

The link generated with

@Html.ActionLink("my link", "Details", new { hash=ViewBag.MyHash })

will have the {hash}-part of the URL lowercased as well, for example if ViewBag.MyHash = "aX3F5U" then the generated link will be /foo/ax3f5u instead of /foo/aX3F5U

Is there a way to force MVC to only lowercase the controller and action parts?

For older versions of MVC, the way to go seemed to be to implement a custom subclass of Route, however I don't know how/where to instantiate it, since the signature of the route constructors is quite different to MapRoute and I'm hoping there to be a simpler way.