ASP.NET MVC Editor-Templates/UIHint with parameters

You could use the AdditionalMetadata attribute:

[UIHint("DateTime")]
[AdditionalMetadata("foo", "bar")]
public DateTime Date { get; set; }

and in the template:

@ViewData.ModelMetadata.AdditionalValues["foo"]

so if you wanted to pass an url:

[UIHint("DateTime")]
[AdditionalMetadata("controller", "somecontroller")]
[AdditionalMetadata("action", "someaction")]
[AdditionalMetadata("property", "someproperty")]
public DateTime Date { get; set; }

and in your template:

@{
    var values = ViewData.ModelMetadata.AdditionalValues;
}

<script type="text/javascript">
$('.auto').autocomplete({
    source: function (request, response) {
        $.ajax({
            url: '@Url.Action((string)values["action"], (string)values["controller"])',
            dataType: "json",
            data: {
                filter: request.term
            },
            success: function (data) {
                response(
                    $.map(eval(data), function (item) {
                        return {
                            label: item['@values["property"]']
                        }
                    })
                );
            }
        });
    }                    
});
</script>

You could use the UHint without AdditionalMetadata attribute, but some additional code is requred

[UIHint("DateTime", null, "key1", "value1", "key2", "value2")]
public DateTime Date { get; set; }

override CreateMetadata:

public class CustomMetadataProvider : DataAnnotationsModelMetadataProvider
    {
        public const string UiHintControlParameters = "UiHintControlParameters";

        protected override ModelMetadata CreateMetadata(
            IEnumerable<Attribute> attributes,
            Type containerType,
            Func<object> modelAccessor,
            Type modelType,
            string propertyName)
        {

            ModelMetadata metadata = base.CreateMetadata(
                attributes,
                containerType,
                modelAccessor,
                modelType,
                propertyName);

            IEnumerable<UIHintAttribute> uiHintAttributes = attributes.OfType<UIHintAttribute>();
            UIHintAttribute uiHintAttribute = uiHintAttributes.FirstOrDefault(a => string.Equals(a.PresentationLayer, "MVC", StringComparison.OrdinalIgnoreCase))
                                              ?? uiHintAttributes.FirstOrDefault(a => String.IsNullOrEmpty(a.PresentationLayer));
            if (uiHintAttribute != null)
            {
                metadata.AdditionalValues.Add(UiHintControlParameters, uiHintAttribute.ControlParameters);
            }

            return metadata;
        }

Register CustomMetadataProvider:

    public static void Application_Start()
    {
        ModelMetadataProviders.Current = new CustomMetadataProvider();
    }

and in your template:

    @{
        IDictionary<string, object> values = (IDictionary<string, object>)
ViewData.ModelMetadata.AdditionalValues[CustomMetadataProvider.UiHintControlParameters];
    }