There's less room for that than you might think - the RTT in the photo is mounted about as far back as it can be. The distance between the front of the RTT and where the back of the rear seat headrests are is probably a foot. If you were to make a hatch large enough to climb through it would need to me more than a foot from front to back, so you'd need to lean the front seats forward in order to access the entire opening. Also unless you're a gymnast, I think you'd need some sort of steps/ladder inside to get yourself up and out so you'd need to figure out how to mount them.
And if you have a longer rack, like this Garvin, the rack reaches all the way to the front seat so an access hatch in the back seat would be useless.
There is already a hatch in the factory hardtop though... how about just removing the Freedom Panels and standing on the front seat?