The Wnck tasklist used in 4.6 had this, and in 4,4 netk too. However during the rewrite to cleanup the code, the old system was not usable anymore, so it need to be redone. There were also some unexpected behaviors in the old grouping mode, that need to be worked on.
This seems to be implemented (and if not, the description is too vague for us to meaningfully figure it out). @nick: Feel free to reopen if you don't agree.