public abstract class QuestionAnswerInputModel {
    public Guid QuestionId {
        get; set;
    }
}

public class RatingQuestionInputModel : QuestionAnswerInputModel{
    [Required]
    [Range(1,4)]
    public int? Rating { get; set; }
}

public class FreeTextQuestionInputModel: QuestionAnswerInputModel{
    [Required]
    public string FreeText { get; set; }
}

public class QuestionAnswerModelBinder : DefaultModelBinder {
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {

        QuestionAnswerInputModel model;

        if ((typeof(QuestionAnswerInputModel) != bindingContext.ModelType)) {
            return null;
        }

        ModelBindingContext context = new ModelBindingContext(bindingContext);

        Type typeOfModel;

        string prefix = bindingContext.ModelName;
        if (bindingContext.ValueProvider.ContainsPrefix(prefix + "." + new FreeTextQuestionInputModel().GetPropertyName(m => m.FreeText))) {
            typeOfModel = typeof(FreeTextQuestionInputModel);
        } else if (bindingContext.ValueProvider.ContainsPrefix(prefix + "." + new RatingQuestionInputModel().GetPropertyName(m => m.Rating))) {
            typeOfModel = typeof(RatingQuestionInputModel);
        } else {
            return null;
        }

        context.ModelMetadata = new ModelMetadata(new DataAnnotationsModelMetadataProvider(), bindingContext.ModelMetadata.ContainerType, null, typeOfModel, bindingContext.ModelName);
        return base.BindModel(controllerContext, context);
    }
}

new DefaultModelBinder().BindModel(controllerContext, context)