The thing that troubles me about rear fogs is that on casual observation they just look like shorted out brake lights. It'd be nice if there was a really conclusive way to differentiate them from regular brake lights.
I think you missed the point. Its not about when the brakes are actually applied, its about looking like one brake light is always on for some reason. That reason is fairly commonly a short. The worst case scenario for a single brake light on all the time would be someone who is already a moron for having only 1 working light left of the 3 they should, and they're riding it all the time. I see lots more of the former than the latter. In Oregon we can get a great deal of fog, so I see the value of having rear fogs.
I like the idea of turning the reflectors into rear fogs on the current gen. That puts them well away from the regular brake lights, they're small enough to not so easily be confused with brake lights, and they're a good marker for the full width of the vehicle. I would put them in BOTH units though, not just one side.