@ G. — Yes, airlines do open additional award space as the departure date approaches all the time. (onemileatatime.boardingarea.com)
In addition, cardholders can access «hidden» additional award space on United flights, which expands your options for redeeming those hard - earned miles. (creditcards.com)
But if you're one of the lucky few, Amex is not doing much in the way of confirming award space, instead relying on an honor system and verbal guarantee from customers in situations where you may claim to find additional award space (as a benefit of elite status) than they can see. (travelcodex.com)