So, as a work around?

function message_to_placeholder_replace($output) {
	if (is_page('messages')) {
		$replace = array('#placeholder="Name of the recipient."#');
		$string = 'placeholder="To"';
		$output = preg_replace($replace, $string, $output, 1);
		return $output;
	}
}
function start_message_to_placeholder_replace() { 
	ob_start("message_to_placeholder_replace"); 
}
add_action('wp_head','start_message_to_placeholder_replace');