mine has used the centre brake light as the fog light and never failed an mot!!you cant use the centre light as a fog. Its an mot fail, and makes you look like a motorcycle.
2 amp general servive diode should be fine, but i would go bigger so it dont run hot. It will need two though. One to protect both circuits or they will have two fuses powering them which is beyond the wireings ability
A friend of mine who ran a eunos import for about 10 years before he bought a mk3, who was an mot tester now retired, had his centre brake light wired up solely as a fog. He said its all about the interpritation of the wording in the mot manual, he said a car must have a working brake light on both sides of the car and if a high level break light is fitted then it must work, his interpritation was that if the high level break light was wired as a fog, it was not a break light, and so did not have to function as such. And to quote him he said " I never failed my car for it"Tisnt an Mot fail, how neat to the centres are the jap import ones?!!, And several cars with centre ones on and centre ones in rear screens aswell, but I would go with either the bootlight or the reverse light but would only have single use, a lot of faffing about to use as both still..and I'll challenge anyone to tell me in thick fog by seeing the fog light they can tell it's a car or a bike due to position when you can't even see the vehicle!