I'm sure that you could fit aftermarket door closure switches to achieve rear light switching only from the rear doors - but that would just be adding some extra potential rust starting areas.
I did that in my 1991 VX Cav GSI 4X4, a lot easier to achieve with an older car, though even with that one I had to change all the rear plunger switches to from single contact to dual contact to retain the benefit of the factory fitted intruder alarm, I seem to remember adding in diodes to the second switch contacts for some reason.
It is just a pity that we can not split up the switching of these front and rear lights when front and rear are fitted, you are lucky not to presently have a rear light, so starting from scratch and using aftermarket door closure switches should work well.
Edit:- I had to cut openings in the side upper frames to fit the lights into, one on each side, you will probably manage to feel your way around to reveal where the roof stiffener opening is - or will they just fit into the head lining if it is stiff?
