OK you guys definitely know your stuff. I was perusing the official 1989 Megadrive/Genesis software guide and came across a few tidbits that relate to this. I took some screenshots.
It seems to me that its also possible to change the display scroll window size and dimensions. It would make sense since they would want to have the capability to render and display a vertical game (like a shooter). They mention that in the manual. Although I don't believe sega made any vertical (display) games for the genesis, although maybe they did in Japan. But such games wouldn't display properly unless paired with a wide CRT (put vertically in a cabinet) which they had back in the day for special arcade games. According to the manual, the machine renders the game (mostly) on a 512x512 square, but only 320x240 is displayed (the display window). This is to account for scrolling or moving to the right horizontally, or going up down vertically. Obviously you know this but, I wonder. You said it may only work in a emulator if its possible and that probably is true, but the hardware (from what I've read) may have been technically capable, even though they never used it. I mean everyone used a standard 4/3 display TV in 1989 so it would make sense not to bother with that feature. (480i 8x16v + vert display)
I honestly don't know enough to make total sense of it, hence I'm forwarding this info to more capable individuals.
LOL the rabbit hole deepens.