Ok, it sort of works on 1.2 experimental, but at least I still need to use freelook to turn my head enough to find the hotspots to activate the change, and not all of the seats are available (even when empty) depending on the gunner's orientation in relation to the vehicle.

Here is a suggestion for further improvement. Have a key that holding it down would bring up all available seats inside the vehicle as a radial menu which would be used to move within the vehicle. That would eliminate the need to find the rather small hotspots to make the seat changes.
You could even assign that to the exit vehicle key assuming that exiting a vehicle would be changed from single press to either hold button action and/or use of radial menu, and that would also allow easy way to exit vehicles to multiple directions when possible (BTR left/right doors).

Another issue is that when you exit the vehicle from the gunner position, you just appear to the outside of the vehicle to the left, so there was no need to open doors for that. I understand that if all the seats were occupied in the car getting out from the gunner position would be possible only through the roof, but without any animation that exit just happens too fast compared to getting out from any of the seats.

And the crouching in to the vehicle to avoid fire feature is still missing, even though I had some glitches that looked like that when I was testing it out. Hopefully you will find the time to add this feature.