cross domain posts to ASP.Net MVC app

I'm developing an app where HTML and javascript chunks are delivered down to different clients. I'm able to GET the html/javascript chunks by adding the following to web config file:

  <system.webServer>
<modules runAllManagedModulesForAllRequests="true" />
  <httpProtocol>
      <customHeaders>
          <add name="Access-Control-Allow-Origin" value="*" />
          <add name="Access-Control-Allow-Headers" value="Content-Type" />
          <add name="Access-Control-Allow-Methods" value="POST, GET, OPTIONS" />
      </customHeaders>
  </httpProtocol>

This is working great for doing GETS. The problem I'm running into is doing POSTs cross domain using jQuery:

        $.ajax(
    {
        type: 'POST',
        url: url,
        crossDomain: true,
        data: JSON.stringify(data),
        dataType: 'json',
        contentType: 'application/json',
        success: function(responseData, textStatus, jqXHR) 
        {
            alert('Success');
        },
        error: function (responseData, textStatus, errorThrown) 
        {
            alert('POST failed.');
        }
    });

I will have numerous clients consuming my app (hopefully). I thought about using a proxy, but I do not have control of the client servers so I'm not able to install a httpHandler to act as a proxy.

Any suggestions on how I can POST json data from different clients cross domain to my ASP.Net MVC app?