I'll go you one better on the doubled fog lights idea (I lived in yurrup for almost 2 decades and believe rear fogs are useful if you live in a foggy area, but I didn't). I did all that you suggest except the diodes, on my 1991 940T. Then I wired the rear fog light wire to the backup lights. I even marked this all back in the covered bulb access with a helpful post-it note for the next owner.

So I could switch on the backup lights by using the fog light switch. Helpful in some scenarios including tailgaters.

The only drawback other than loss of rear fog lights was that when I put the tranny in reverse, the fog light indicator on the dash (or switch, I forget which) would come on. Not a big problem.

If you really want rear fogs too you could get a surface mount trailer LED and wire it up. But then, you could do the same for doubled brake lights. Those surface mount trailer LEDs didn't exist back in 1991.