Coming from Perforce, this is something I really appreciate. Select a bunch of things, make a new CL out of them with a clear name. This way if I ever need to push them later, I have a bundle of things ready to go. This is a good way to keep your private things organized for you and out of the way of other people.
This would be a list still present in your pending window, but a collapsible group/reorderable list of collapsible groups at the bottom of the pending, just like in Perforce.
Having to go into the ignore.conf file to really manage files efficiently seems to go against Plastic's simplified route (even then you can't group items scattered around the project), but currently it's doing so at the cost of functionality which is 100% worse than having complexity in the UI.