I have past dated my tickets to shows I couldn't make in the past many times, but through Telecharge, not Ticketmaster.
Yes, they can tell the ticket hasn't been used by the bar code.
With Telecharge at least, you have to call on the day on the performance to see if they will be allowing past dated tickets that night. Sometimes you have to call many times before they say they will accept them.
"Winning a Tony this year is like winning Best Attendance in third grade: no one will care but the winner and their mom."
-Kad
"I have also met him in person, and I find him to be quite funny actually. Arrogant and often misinformed, but still funny."
-bjh2114 (on Michael Riedel)