public/lib/select2-search-placeholder.js

(function($) {

    var Defaults = $.fn.select2.amd.require('select2/defaults');

    $.extend(Defaults.defaults, {
        searchInputPlaceholder: '',
        searchInputCssClass: '',
    });

    var SearchDropdown = $.fn.select2.amd.require('select2/dropdown/search');

    var _renderSearchDropdown = SearchDropdown.prototype.render;

    SearchDropdown.prototype.render = function(decorated) {

        // invoke parent method
        var $rendered = _renderSearchDropdown.apply(this, Array.prototype.slice.apply(arguments));

        this.$search.attr('placeholder', this.options.get('searchInputPlaceholder'));
        this.$search.addClass(this.options.get('searchInputCssClass'));

        return $rendered;
    };

})(window.jQuery);