Hey, sorry for the super late response, but: buyers do get notified on "new files" for items that they've bought. So, pre-orders work the same way - if they try to download while the placeholders are there they'll be told to wait for the game to be ready, and once the actual files are uploaded/published, they'll get an e-mail saying "there's new uploads for a game you own".
Additionally, you can send a custom e-mail to all your buyers from the Edit game -> Interact -> Email tab.