CiviEvent Access Code on price set
This is an interesting use case!  I can see the advantages over the approval workflow.

If I were asked to do this, I'd probably:


Create a custom field "approval code" on the event;
Use hook_civicrm_buildForm and jQuery to hide the page except for an approval code submission field;
When the entered code matches the stored value, show the page.


This isn't completely foolproof if you rely too much on client-side jQuery though.

If you decide to go the price-set route, you might want to use CiviDiscount instead.  The "discount" code can be the approval code.