Like Brandon I have never heard of overplugging but I have heard the feature you describe called "Camp-On" on legacy PBX's.
It is not a standard feature of CUCM 8.x but there is athird party solution that offer support for it.
Radianta Beacon Office offers a feature called Personal Queuing that can do what you need
It also has other features such as paging, call recording and more.
If they are willing to modify the way in which they work then the callback feature included in CUCM may provide the functionality they need at no cost.