This is just a shot in the dark, completely untested.
Plugin file:
<?php
class placeholder extends rcube_plugin {
public function init() {
$this->include_script("placeholder.js");
}
}
JS file:
$('input[name="_user"]').attr('placeholder', '<username>');
$('input[name="_pass"]').attr('placeholder', '<password>');
Replace the <username> and <password> in the JS file with the placeholders you want.