Hi Tom,
Here's what I am thinking about:
Create a PM Group which will be constantly updated every time a new Project Manager is assigned to a Project, this can be done using a Workflow Rule that runs on Projects creation, checks if this user is not a member of that group and adds him to the group.
Then, your rule will notify this group every time the workflow runs.
You can add an additional flag to only notify Project Managers in that group that are of a specific type if you don't want to send a notification to all PMs or alternatively, come-up with a criteria to define if to add or not add that user to this group based on a particular attribute of the Project.