[Gary MichaelSupport Manager]

Okay, try this:

add_filter( 'syncs3_s3urls_merge_tag_link_anchor_text', 'em_replace_s3url_link_text', 10, 4 );
/**
 * Modify a link's anchor text in the {s3urls} merge tag.
 *
 * @param  string 	$text  	Anchor text
 * @param  string 	$url   	File S3 URL
 * @param  array 	$form  	Form data
 * @param  array 	$entry 	Entry data
 *
 * @return string
 */
function em_replace_s3url_link_text( $text, $url, $form, $entry ) {
	return "C:/path/to/{$text}";
}

The filename is now the default, so this is just prepending the path. All you need to do is adjust the path in front of {$text} – the {$text} portion is the filename.