Couple thoughts:

I'm +1 for adding TaskGroup.cancel() method.

I'd be -1 on abusing `Task.cancel()` to signal something with a nonce. Whatever problem we are trying to solve here, it should be solvable without resorting to hacks like this. It should be trivial to implement simple tracking of whether a child task was cancelled by the group or not to decide on how to handle a rogue CancelledError.