There is no system working like that, InventoryUpdate does not load items from database.
You have to make a script checking some database table every X minutes and send item when added, or make sure player is offline.
You can also use telnet to send items, see scripts/handlers/telnethandlers/player/SendMail.java
As far as InventoryUpdate works.
final InventoryUpdate iu = new InventoryUpdate();
iu.addItems(L2ItemInstance list);
player.sendInventoryUpdate(iu);