I'm not sure this is the correct way to handle this problem, and the solution is from a EPi 6 project. We created created a new URL-rewriter (might work to attach to an event instead, but we had some other functionality the needed a own rewriter), and in that rewriter we did: 

protected override bool ConvertToExternalInternal(EPiServer.UrlBuilder url, object internalObject, System.Text.Encoding toEncoding)
        {

            if (!PageReference.IsNullOrEmpty(internalObject as PageReference))
            {
                var page = DataFactory.Instance.GetPage((PageReference)internalObject);
                if (page != null && page.LinkType == PageShortcutType.Normal && page["PageExternalURL"] != null)
                {
                    url.Path = "/" + page["PageExternalURL"];
                    url.QueryCollection.Remove("id");
                    url.QueryCollection.Remove("epslanguage");
                    return true;
                }
            }

            return base.ConvertToExternalInternal(url, internalObject, toEncoding);
        }

The above solution Erik suggests will work for WebForms but not for MVC (since MVC responoses does not pass through the FURL parsing). Another alternative is to attach an eventHandler to EPiServer.Web.Routing.ContentRoute.VirtualPathCreated. In the eventhandler you could load the content item and see if it has an simple address and if so replace the Url in the event argument (similar to the code above).