I've actually been in the maintenance room on a tour a couple years ago.
It looks like a fairly normal professional garage and if I remember correctly takes up the bottom northwest corner of the show building with access to the parking garage and service alleys off of Cypress dr. It was fairly small too- with only space for two vehicles to be worked on at once, and I believe vehicles can be switched to manual control via a cable remote and "walked" outside of the bay to make more room. Since all the vehicles are trackless, the computer simply tells them when to go to maintenance and the trundle down the corridor to the bays by themselves. I remember seeing black marks on the ground in and near the garage where the vehicles' wheels had run over the same paths, year after year.
Looking at the diagram, I assume, but don't know for certain that the maintenance corridor is just an offshoot of the 1st floor (yellow) on Martin's diagram, going off to the left.
I'll add a couple points:
There are two maintenance bays for Tower, one on each side. I've also seen lights-on photos showing that there are some tool cases and workbenches on the Unload level next to the Load shafts. Presumably it's so they can do simple maintenance and inspections on the vehicles when the ride is powered down rather than pull them into the maintenance bay.
Seatbelts do not have to be buckled when the vehicle leaves Unload. It's up to the CM working the Unload point to make sure the vehcile is clear of people and objects before they press the button that sends the car on its way. The ride system takes care of it from there.
Guys do you know how the AGV (I think its called that) gets secured in to the cage that moves up and down??