I believe the best way to do it, is - as described in your links - to extend ActionResult or extend JsonResult directly.

As for the method JsonResult that is not virtual on the controller that's not true, just choose the right overload. This works well:

protected override JsonResult Json(object data, string contentType, Encoding contentEncoding)

EDIT 1 : A JsonResult extension...

public class JsonNetResult : JsonResult
{
    public override void ExecuteResult(ControllerContext context)
    {
        if (context == null)
            throw new ArgumentNullException("context");

        var response = context.HttpContext.Response;

        response.ContentType = !String.IsNullOrEmpty(ContentType) 
            ? ContentType 
            : "application/json";

        if (ContentEncoding != null)
            response.ContentEncoding = ContentEncoding;

        // If you need special handling, you can call another form of SerializeObject below
        var serializedObject = JsonConvert.SerializeObject(Data, Formatting.Indented);
        response.Write(serializedObject);
    }

EDIT 2 : I removed the check for Data being null as per the suggestions below. That should make newer versions of JQuery happy and seems like the sane thing to do, as the response can then be unconditionally deserialized. Be aware though, that this is not the default behavior for JSON responses from ASP.NET MVC, which rather responds with an empty string, when there's no data.