What you can do is create a notification for each member of the oncall rotation and manage the "active" notification using Notification Schedules.

Thank you for your answer! That makes a lot of sense, and I will try to get it working properly with a Schedule for each week of the month.

EDIT: I took a look at the possible schedule options, and I am unclear as to how Zenoss will figure out that a schedule starting on the first Monday of the month, lasting for a week, will repeat itself for the first week of the next month Since the "Start Date" is linked to a specific date, wouldn't the schedule move around forward and backward through the year, making a person switch to guard duty through Tuesday night (for example)

Hmm ok, I never actually used the Notification Schedules but I see your problem. Maintenance Windows on devices have the same issue. Could probably use the API to set/update the notification from a script. Or the tried and true use a mail alias and change the target of the alias somehow...