This really isn't something Daz is going to "fix." If you stop and think about it, what the iRay rendering engine is trying to do is to simulate how light works in the real world. If you were setting up a real photo shoot and put a spotlight in to illuminate something, you wouldn't be surprised if you could see that spotlight in a mirror, would you? That's a direct analogy to what's going on here - the spotlight is emitting light, and if it bounces off the mirror and into your camera, well...
This is one of the reasons the lighting guys on movie sets get paid big bucks - they have to figure out how to make sure everything is nicely illuminated without the "how it's done" showing up in the shot.
BTW, ghost lights (emissives with very low cutout opacity) weren't something that was purposefully designed into the iRay rendering engine - they turned out to be a happy accident that was discovered by users. They're the thing that is artificial - there's no way to do that in the real world. Similarly, the fact that a spotlight isn't (directly) visible from the back is also artificial. But once the light leaves a spotlight, iRay's going to try to do what that light would in the real world.
So this leaves you with two basic choices - either move the light so it isn't visible, or get rid of it in post work. (Which is exactly what the movie guys do - one of the "you never notice it" things special effects artists are frequently doing in movies are taking out stuff that was there that the director doesn't want seen. Support wires, cars in the background, etc., etc. It's a very unsung part of the special effects industry because it's not "make this fantastic creature" work.)
