Workaround to make multi-line custom donation fields appear as <textarea> tags.

multi_line_custom_donation_display.html

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script type="text/javascript">
// See https://salsasupport.zendesk.com/forums/21464331
// Requires: Any fairly modern version of jQuery.
//
$(document).ready(function () {
    // You do this.  Put the API names of the multi-line donation custom fields
    // between the quotes. Separate them with commas.  If there is only one
    // field name, simply type it between the quotes without any commas.
    //
    var field_names = "field_1_api_name,field_2_api_name,you_get_the_idea".split(' ');

    $.each(field_names, function (index, name) {
        var e = $('input[name=' + name + ']');
        if (e.length > 0) {
            var id = e.attr('id');
            var title = e.attr('title');
            var text = [ '<textarea id="' + id + '"',
                'name="' + name + '"',
                'value=""',
                'class="blockInput"',
                'title="' + title + '"></textarea>'
            ].join(' ')
            e.parent().append(text)
            e.remove()
        }
    })
})
</script>