One workaround is to try to opt-out of the model binding of the parameter and use the direct deserialization model.  This can be done by using a IRequestContentReadPolicy.  Say your action is called 'Post':

public class ReadAsSingleObjectPolicy : IRequestContentReadPolicy
    {
        public RequestContentReadKind GetRequestContentReadKind(HttpActionDescriptor actionDescriptor)
        {
            // deserialization model
            if (actionDescriptor.ActionName.Equals("Post"))
            {
                return RequestContentReadKind.AsSingleObject;
            }
            else // default model binding
            {
                return RequestContentReadKind.AsKeyValuePairs;
            }
        }
    }

Here is how you can add this IRequestContentReadPolicy in the config:  

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

We'll try to fix this in the next release so that this scenario would just work.  

Thanks,
Maggie Ying