This is the information from the WDW Guidebook for Guests with Disabilities (there is a link in the disABILITIES FAQs thread; this is from the "Mobility Disabilities" link).
There is an outside location listed for AK (the Outpost, marked with
bold type on the list below), but none of the outside of the park places list
ECVs; just wheelchairs. Since you are planning to be there before opening, you could get a wheelchair there and trade it in for an
ECV inside the park.
It is a pretty long walk from the parking area to the Outpost; there may be some wheelchairs that can be borrowed to get there from the handicapped parking to the wheelchair/
ECV rental area, but no guarantees that one will be available.
Someone else can probably confirm, but my understanding is that the wheelchair and ECV rental areas are open as soon as they open the park to let guests in (which would mean they would be available for early morning ADRs.)
Magic Kingdom® Park (the outside the park areas are
underlined
Frontier Trading Post in Frontierland® (replacement location)
Mickey's Gift Station at Ticket and Transportation Center (limited quantities)
Stroller and Wheelchair shop inside Main Entrance (also rents ECVs)
Tinkerbell's Treasures in Fantasyland® (replacement location)
Tomorrowland Arcade (replacement location)
Epcot®
Gift Stop just outside Main Entrance (limited quantities)
Glas und Porzellan in Germany Pavilion (replacement location)
International Gateway (also rents ECVs)
Stroller and Wheelchair location inside Main Entrance (also rents ECVs)
Disney-MGM Studios
Oscar's Super Service inside Main Entrance (also rents ECVs)
Tatooine Traders at Star Tours (replacement location)
The Writer's Stop (replacement location)
Disney's Animal Kingdom® Theme Park
Creature Comforts (replacement location)
Garden Gate Gifts inside Main Entrance (also rents ECVs)
Mombasa Marketplace (replacement location)
Outpost (limited quantities) is to your left before you enter the park.