If it helps my custom binder to solve the issue is:

kendo.data.binders.widget.buttonText = kendo.data.Binder.extend({
    refresh: function () {
      var text = this.bindings.buttonText.get();
      if (text == null) {
        text = "";
      }
      $(this.element.element).contents().filter(function() {
        if (this.nodeType == Node.TEXT_NODE) {
          this.nodeValue = text;
        }
      });
    }
  });