Ah Smitty, I like ALL buses, no matter the style!
I think i would just mount the ladder on the rear of the bus with a flip down section that reaches the ground for the tipsy drunkards & anyone of any size can fit on it! This was it is out of the way & you are not taking up valuable space on the inside of the bus along with the foot traffic everytime someone has to take a leak... OR just pee offa the roof!
Plus putting a hatch on the curved section of the roof is not going to be an easy task...& then there is possible leakage to also consider.