Alan D. was nice enough to send me an email with the scan above attached. How awesome is that postmark? I definitely think it is worth sending out for. If you are interested, send your already stamped mail (inside another envelope) to:
West Point Post Office
Pearl Harbor cancellation
634 Swift Road
West Point, NY 10996
I couldn't find anywhere online that specifically said so, but generally special cancellations are available for 30 days after the event they were created for.