You can listen to the video-playback.CHANNEL_NAME topic to receive this information. Just keep in mind that it is not supported, as far as I can tell anyway.
{
"type": "LISTEN",
"data": {
"topics": ["video-playback.CHANNEL_NAME"],
"auth_token": ".."
}
}
You’ll receive a message similar to this once the channel goes online. When the channel is offline no information is sent.
{
"type": "MESSAGE",
"data": {
"topic": "video-playback.lirik",
"message": "{\"type\":\"viewcount\",\"server_time\":1499721302.779628,\"viewers\":31846}"
}
}
Hope this helps!