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>