There is a filter at the end of the function: 'get_sample_permalink_html'. Hook into this and just replace the shortened form with the full length.
<?php # -*- coding: utf-8 -*-
/* Plugin Name: T5 Unabridge Permalink Slug */
add_filter( 'get_sample_permalink_html', 't5_unabridge_sample_permalink', 10, 2 );
/**
* Replaces the shortened permalink with its full form.
*
* @param string $sample Permalink HTML
* @param int $id Post ID
* @return string
*/
function t5_unabridge_sample_permalink( $sample, $id )
{
$link = get_sample_permalink( $id );
$s1 = '<span id="editable-post-name" ';
$s2 = '</span>';
return preg_replace(
'~' . $s1 . '([^>]*)>([^<]*)' . $s2 . '~Ui',
$s1 . '$1>' . $link[1] . $s2,
$sample
);
}
Result
Post title: This is a rather long post title. WordPress would shorten it by default, but our nice plugin prevents that.
