yes, that's it:

"Create a second surface slightly behind the mirror of approximately the same shape. Doesn't have to be exact. Set this up as a diffuse reflector however you like - perhaps just plain white with DV = 100%.

Then set the actual mirror to be invisible to ray-tracing. Assuming you don't need to see the mirror in another mirror, it would work."

one might also mention they used to fake this for spotlite/atmosphere in old version of carrara by putting virtual spotlite behind mirror and negative shadow, but I haven't tried anything like this in poser, altho negative shadows and shadows >1 worked as recently as poser 8 IIRC.