This idea crossed my mind when I was attracted to picking up something like a GFC but wanted to be able to use whatever I bought on both my Tacoma and my Range Rover Classic(even if the pass through would only be useful on the Taco), but then it crossed its way right back out again for a few key reasons. The issue stems from the fact that the structure for a roof top tent is in the floor because they are designed to be supported by horizontal load bars, while something like a GoFastCamper can be designed from the ground up with the structure around the outside edge. Take the Eezi Awn Series III for instance as the structure on the bottom of the tent is exposed and easy to see, if you look at the bottom of the 1400 model there is a spot where you could cut out a pass through but it would be pretty impractically small to the point it wouldn't really be useful for anything besides slipping down into the bed to grab something and maybe making it slightly easier to put pants on in the morning. Larger models have additional supports and provide no spot to cut a pass through. For me at least the occasional trip down a ladder to get something and dressing horizontally aren't such big pain points that they need addressing, the real appeal of a GFC or similar is that they provide a proper space where you could cook or hang out during inclement weather. Chopping a hole in the bottom of a rooftop tent would give me at best a poor imitation of that and do a poor job of being modular because whenever the tent's off the Tacoma I'd have a gaping hole in my topper to fill which would never be particularly watertight unless I engineered a gutter system similar to a sunroof and that was where I drew the line of the project being pointlessly complicated.