We had 8:05 reservations for Crystal Palace in MK and we got there via Disney buses. The Disney buses are running really early just for this reason. We took the MK bus and they stopped at MK, Contemporary (for Chef Mickey) and Poly (for Ohana). I think we were on the bus by 7:15am.
I am not sure exactly what time the buses start running in the morning but when I went to check out at Pop just this past Saturday morning, the MK bus pulled up and picked a family up at 6:30 am.
Tyler on the transportation board is very helpful with this type of information. I bet if you post over there you will get more responses.
Hope this helps!