Just like mass delete, this should have numerous "mass read" options to cut down on the number of unread notifications active contributors receive.
Maybe like this:
- mark all notifications by <username> as read
- mark all notifications at <page name here, optionally a glob mask> as read
- mark all notifications from <time 1> to <time 2> as read
- mark all notifications of type <mention, email, etc> as read
- combination of any of the above