Soft and Hard Deadlines for Software updates
Software updates need a 'hard' deadline so that computers who have hit the updates but never leave their computers on within a maintenance windows, will never have the updates installed.
1st : Deployments made available for users to install at their leisure from the software center. Possible
2nd : After say 1 week, install the updates as per maintenance windows in the evening. Possible
3rd : After 1 more week if not yet installed, just install no matter what at any time, with the standard notifications for reboots as per SCCM client settings. "Currently not possible"