I think you’ll find the closest thing to what you want to achieve is the undocumented chatters endpoint: http://tmi.twitch.tv/group/user/CHANNEL_NAME/chatters
This will give you a list of users who are currently logged in to chat for that channel. Like all endpoints it is cached so wont be super precise, and the users listed might just be in chat but not even have the stream open. Also, as an undocumented endpoint it might be removed/changed/break at any point.