Hi,

this might not be exactly what you are looking for but we have the same field "required" through validation when using it.
Create a validator using the IValidate interface, use it to validate content (this might be preformance heavy but it works fine on small-medium sites). 

Something like this, Ex:

IEnumerable<ValidationError> IValidate<ContentData>.Validate(ContentData page)
        {
            string errorMessageTemplate = @"Bilden med namn: ""{0}"" saknar beskrivning!";
            ValidationErrorSeverity severity = ValidationErrorSeverity.Error;
            List<ValidationError> errors = new List<ValidationError>();
            PropertyDataCollection pageProperties = page.Property;

            foreach (PropertyData propertyData in pageProperties)
            {
                //Built-in properties that we want to ignore
                if (propertyData.OwnerTab == -1) continue;

                try
                {
                    //ContentArea
                    if (propertyData.PropertyValueType == typeof(ContentArea))
                    {
                        var contentArea = (ContentArea)propertyData.Value;

                        if (contentArea.Items != null)
                        {
                            foreach (var item in contentArea.Items)
                            {
                                var content = item.GetContent();

                                if (content as ImageFile != null)
                                {
                                    if (string.IsNullOrEmpty(((ImageFile)content).Description))
                                    {
                                        errors.Add(
                                            new ValidationError
                                            {
                                                ErrorMessage = string.Format(errorMessageTemplate, ((ImageFile)content).Name),
                                                PropertyName = propertyData.Name,
                                                Severity = severity
                                            });
                                    }
                                }
                            }
                        }
                    }
                    //ContentReference
                    else if (propertyData.PropertyValueType == typeof(ContentReference))
                    {


                        var reference = propertyData.IsNull ? null : propertyData.Value as ContentReference;// page.GetValue(propertyData.Name) as ContentReference;

                        if (!ContentReference.IsNullOrEmpty(reference))
                        {
                            ImageFile file = null;
                            var locator = ServiceLocator.Current.GetInstance<IContentRepository>();

                            try
                            {
                                file = locator.Get<ImageFile>(reference);
                            }
                            catch (Exception)
                            {
                                file = null;
                            }

                            if (file != null)
                            {
                                if (string.IsNullOrEmpty(((ImageFile)file).Description))
                                {
                                    errors.Add(
                                            new ValidationError
                                            {
                                                ErrorMessage = string.Format(errorMessageTemplate, file.Name),
                                                PropertyName = propertyData.Name,
                                                Severity = severity
                                            });
                                }
                            }
                        }
                    }
                }
                catch
                {
                   //Handle
                }

            }

            return errors;
        }

Should give you a view like this when trying to use an image without description: