For SVA can I use a LED brake light ring (separate to the main LED Stop/Tail lights ) wired to just show the brake light LED's as a rear fog.

Doubtful.
Although the bulb intensity is the same (led in your case) the fog light has a silver reflector built in that the brake light does not. Also remeber a fog has to be 100mm min away from a brake light.
Depends how savvy your sva dude is on the day.

aren't the latest audi ones (R8 at least) LED fog lamps? they look the same as the 3rd brake light, just lower down

Not sure about Audi's but the latest Subaru Legacy's rear fog is an LED unit.