sansys.net: http://goo.gl/92CHdD

public class CustomDataHandler: UrlRoutingHandler
{
    protected override void VerifyAndProcessRequest(IHttpHandler httpHandler, HttpContextBase httpContext)
    {
        RouteData routeData = httpContext.Request.RequestContext.RouteData;

        httpContext.Response.ContentType = "text/plain";
        httpContext.Response.Write(routeData.Values["dataKey"]);
    }
}