I dunno, it was always my impression that when you describe someone as 能面, it was more-so emphasizing the fact that the person doesn't show any emotion (a mask never changes, and generally masks are in a very generic expression). Though 能面野郎 has a lot of hits on Google (blogs and twitter, mostly), I couldn't find any specific explanation of the term, so I can't tell you for sure if there's a special meaning to it.
My gut instinct, though, would be that they're calling the person "An emotionless bastard." If you're gonna talk about backstabbing, Japanese has plenty of more colorful terms. =D