How about a sign in the header next to the chat that reads "LIVE VIDEO CHAT RIGHT NOW!" or something like that? Would maybe attract more members who missed the topic for example.
I know that you have said that you are planning on doing a kind of schedule for video chats. When that happens there could be a countdown on the same place as the sign would be. "Video chat coming up in: XX minutes"
Just some suggestion, thought they would be cool