There’s the undocumented chatters endpoint http://tmi.twitch.tv/group/user/CHANNEL_NAME/chatters that you could use. This is NOT a list of viewers though, it’s a list of people connected to chat and in that channel, they may not be watching the stream (but this fact also remains with things like JOIN/PART messages too). It’s also cached like all endpoints so it’s accuracy may or may not suit your use case.