Sure, Snaz grabs it off the Twitch API as far as I remember.
Question is if the thread author wanted it for himself ingame or for the viewers on stream only...I dont think snaz has such an overlay feature and it would be quite complicated to do anyway (without breaking hooks/games/anti-cheat mechanisms, etc).
You and everyone else is free to write a plugin that shows your Twitch Viewer Count on stream or as an overlay, if you find a way to do that.