The reason a BOV vents is because the throttle plate closes creating a difference in pressure between the manifold and the intake tract before the throttle.
With an auto, because you shift without lifting off the throttle, this pressure difference never exists, so the BOV won't vent. Same reason diesels don't have BOVs.
You could deliberately make it vent by lifting off the throttle very suddenly, but it seems like a pointless exercise.