Model binding would not work in this kind of scenario. We have provided a flag to go through a de-serialization approach for these kind of scenarios. The whole body is deserialized as it used to in WebAPI preview 6.

public class ReadAsSingleObjectPolicy : IRequestContentReadPolicy
    {
        public RequestContentReadKind GetRequestContentReadKind(HttpActionDescriptor actionDescriptor)
        {
            //Serialization approach
            if (actionDescriptor.ActionName.Equals("PostDC_PersonAsKeyValuePair"))
            {
                return RequestContentReadKind.AsSingleObject;
            }
            else //Default
            {
                return RequestContentReadKind.AsKeyValuePairs;
            }
        }
    }

It can be configured on service's HttpConfiguration object as 

config.ServiceResolver.SetService(typeof(IRequestContentReadPolicy), new ReadAsSingleObjectPolicy());