I like this idea. Last night I was wishing it was there...
As for the "each channel lacks moderation" so do the 1v1 private channels.
What's the difference? A few more people? It's all the same, really...
On top of that, it would be "private" so it would only bother those who have entered the chat.
If it weren't for the multiplayer chat channels, I would think this was an impossible idea.
Isn't there a way to set up a normal channel the same way, except without having to be in multiplayer (so if you wanted to watch someone play and have a group talk about it, or discuss a map in its creation stages with multiple people without bothering the OSU channel)
Maybe you could put a limit on it for up to 8 people like multiplayer or something?