IME with expiring miles I've used the mall, even if the miles don't go through instantly (it officially can take 6-8 weeks or something like that) as long as the purchase was made before the expiration date, even if the miles expire and disappear from your account, they will recredit when the purchase miles post.
there is always the option of booking an award ticket and cancelling it, the only thing you stand to lose is the cancellation/redeposit fee
