And now cgrouproot can live in /proc?

Um, /proc/$pid/task *is* how procfs provides information on threads. It's not 'groups of processes', it's groups of *kernel tasks*, i.e. schedulable entities: what POSIX calls threads. There is nothing in procfs to track groups of processes in any other sense (you can't even follow the pid -> ppid hierarchy via the directory hierarchy or via symlinks, you have to parse /proc/$pid/status).

You really don't know very much about Linux at this level, do you?