Just so I get it straight, employees wait at the gate till the sales close then get a ticket if there are any empty seats?
Not quite. You rock up to check-in, if there is plenty of room they might give you a boarding pass straight away, but more likely it's "thanks, come back in x minutes" (which will be shortly after the flight has closed for check-in and they've run their numbers).
Then if there is room and you can get on, you'll get your boarding pass and can then proceed to the gate. Naturally for international you can't go through immigration and security without a boarding pass.
If there's no room, then you get shifted to try your luck with the next flight, or come back tomorrow.
As a result, it really is not possible for a sub-load staff ticket to prevent a revenue passenger from uplift.