I called and made a PS for 8:10. I believe this is the earliest because the CMs actually escort you to the restaurant. If I'm wrong (and it is possible) correct me.
We have a 8:10 PS for Donald's comming up on a non EM day. I tried to make a 8:00am PS and was told 8:10 was the first seating.
We have done this before and it works out nice. We finished right about 9am, and walk right over to Dinasour. AK is incredible when it's empty!!!
We have also have had 8:10 seatings at Crystal Palace, and got to walk up a empty Main Street... wow.