Using POSM OOS messages for non-POSM-managed products

[torvista]

I did this some time ago, so the same options appear in a drop-down in the product edit page. While revisiting this in a spring clean, I'm using the notifier to get the non-managed product oos message into the vanilla POSM handling for these base products.

$this->notify(
                'NOTIFY_POSM_GET_IN_STOCK_MESSAGE',
                [
                    'prid' => $prid,
                    'pos_record' => $pos_record,
                ],
                $message_override,
                $use_in_stock_message,
                $show_mixed_stock_messages,
                $no_stock_message //steve added
            );

But I don't see how the vanilla oos message text can be overridden without adding the extra parm as shown.