This hack works until I can come up with something better.

1. Create a wrapper around the remote validation method that takes a different parameter name. Forward to the validation method.

In my case I have IsUID_Available2(string FirstName) as the wrapper method with the 2nd field name.

[OutputCache(Location = OutputCacheLocation.None, NoStore = true)]
    public class ValidationController : Controller {


        public JsonResult IsUID_Available(string Username) {


            if (UserNameHelper.IsAvailable(Username))
                return Json(true, JsonRequestBehavior.AllowGet);


            string suggestedUID = String.Format(CultureInfo.InvariantCulture,
                "{0} is not available.", Username);


            for (int i = 1; i < 100; i++) {
                string altCandidate = Username + i.ToString();
                if (UserNameHelper.IsAvailable(altCandidate)) {
                    suggestedUID = String.Format(CultureInfo.InvariantCulture,
                   "{0} is not available. Try {1}.", Username, altCandidate);
                    break;
                }
            }
            return Json(suggestedUID, JsonRequestBehavior.AllowGet);
        }
        public JsonResult IsUID_Available2(string FirstName) {
            return IsUID_Available(FirstName);
        }
    }