I was about to post this same question. I'm arriving early my first day and I was planning on going to AK. I'm staying at BLT and I'd really rather not have to drive all the way to BLT just to turn around and drive to AK. That's, what, 40 minutes of time!
I saw a post on touringplans that suggested bringing your reservation printout to the person at the parking booth, but no validation on whether it'd work or not.