Warlock: you might be right, actually, it could have just been the thing we were testing with was a face, which (if it were the reflection person approaching the mirror) would behave the opposite of the way it works.

We actually did decide to do it with duplication instead because with the idea we ended up with angle mattered, and you can only get a view from one angle with the mirror mapping.  Hopefully the polycount doesn't explode on us ;)