If you're just looking for a Magical Moment not in your wedding attire, then you might want to try asking either concierge at your resort or guest services at Magic Kingdom how you might go about getting that to happen - being extra nice and sweet when you ask.
Or, what might be even better, have your fiance ask someone making it look like he would really love for this to happen as a special wedding present surprise for his new Disney wife. They might be even more willing to try to make it happen if possible or at least point him in the right direction.
Good luck - would be pretty cool if it worked out for you.
However, it sounds like the timing might not work out for you since you said your shoot is until 8:30am and the park opens at 9am, but you would still have to change before getting back to the entrance in time. But if the park opens at 9am, the show would probably start around 8:30-8:45. Maybe you could try it for another day.