I have never tested JGroups on hundreds of machines, as I don't have access to such a big lab. And network simulators only get you that far.
I'd be interested in working with you to make it work if it doesn't.
I know spead but haven't used it. I wrote JGroups to have a 100% Java solution, and no JNI code in it. At one time, I had a bridge (pipe) to an external Ensemble process, so I didn't have to use JNI, but that was not a clean solution. IMO anything that involves external processes or JNI code are not nice.