Ignore the above, I've got it working now, it was a typo! For any one else that needs this, all I did to the password plugin was set $config['password_driver'] = 'cpanel' and $config['password_cpanel_host'] = 'server hostname'. Then in drivers/cpanel.php I changed from this
$data = [
'email' => password::username('%l'),
'password' => $newpass
];
to this
$data = [
'email' => password::username('%u'),
'password' => $newpass
];