public class CustomJsonAttribute : Attribute, IControllerConfiguration 
{
    public void Initialize(HttpControllerSettings controllerSettings, HttpControllerDescriptor controllerDescriptor)
    {
        var formatter = controllerSettings.Formatters.JsonFormatter;

        controllerSettings.Formatters.Remove(formatter);

        formatter = new JsonMediaTypeFormatter
        {
            SerializerSettings =
            {
                ContractResolver = new CamelCasePropertyNamesContractResolver()
            }
        };

        controllerSettings.Formatters.Insert(0, formatter);
    }
}

[CustomJson]
public class MyController : ApiController
{
    public IHttpActionResult Get()
    {
        var model = new MyModel();
        return Ok(model);
    }
}