A channel could add a raw item like io.pnut.core.broadcast-thread as simple as this:
{
"type": "io.pnut.core.broadcast-thread",
"value": {
"thread_id": "2858"
}
}
If a client notices this on a channel, whenever it creates a net.patter-app.broadcast, it would create the Global post as a reply_to this thread_id. Then on Global, the broadcasts would be naturally tied together in that channel's thread.