_config.php

<?php

Config::inst()->update( 'UserDefinedForm', 'extensions', array('UserDefinedFormExtension') );

MFUserForm.js

(function($){
  $.entwine('ss', function($) {
    $('.ss-gridfield .action.userform-export-csv').entwine({
      onclick: function(e){
        var self = this, btn = this.closest(':button'), grid = this.getGridField();          

        var data = '';

        // Add current button
        data += '&' + encodeURIComponent(btn.attr('name')) + '=' + encodeURIComponent(btn.val());

        // Include any GET parameters from the current URL, as the view state might depend on it.
        // For example, a list prefiltered through external search criteria might be passed to GridField.
        if(window.location.search) data = window.location.search.replace(/^\?/, '') + '&' + data;

        window.location.href = $.path.makeUrlAbsolute(grid.data('url') + '?' + data, $('base').attr('href'));
        return false;
      }
    });
  });
}(jQuery));

UserDefinedFormExtension.php

<?php

class UserDefinedFormExtension extends DataExtension {


  public function getCMSFields() {
    $fields = parent::getCMSFields();
    $this->extend( 'updateCMSFields', $fields );
    return $fields;
  }

  public function updateCMSFields( FieldList $fields ) {

    Requirements::javascript('mysite/javascript/MFUserForm.js');

    $reports = $fields->fieldByName('Root.Submissions.Reports');
    $config = $reports->getConfig();

    // remove problematic export button
    $config->removeComponentsByType( 'GridFieldExportButton' );

    // remove print button. Because it sucks too.
    $config->removeComponentsByType( 'GridFieldPrintButton' );

    // add our own!
    $config->addComponent( $export = new UserFormGridFieldExportButton() );

  }
}

UserFormGridFieldExportButton.php

<?php
/**
 * @package framework
 * @subpackage gridfield
 */

/**
 * Adds an "Export list" button to the bottom of a GridField.
 */
class UserFormGridFieldExportButton extends GridFieldExportButton {
  /**
   * Place the export button in a <p> tag below the field
   */
  public function getHTMLFragments($gridField) {
    $button = new GridField_FormAction(
      $gridField, 
      'export', 
      _t('TableListField.CSVEXPORT', 'Export to CSV'),
      'export', 
      null
    );
    $button->setAttribute('data-icon', 'download-csv');
    $button->addExtraClass('userform-export-csv');
    return array(
      $this->targetFragment => '<p class="grid-csv-button">' . $button->Field() . '</p>',
    );
  }

}