Answer
Rather than modifying the system-defined event directly, chain a new custom event off it: create a brand-new custom Event (Action Type = Email) containing whatever recipient-resolution logic is actually needed (e.g. looking up the specific project manager), and enable it. Then, on the original system-defined event, disable its existing Email-type action, and add a new Event Action of type 'Execute Online SQL' that simply calls/triggers the new custom event you just built. The next time the system-defined event fires, it triggers your custom event through that Execute Online SQL action instead of sending its own built-in email — giving you full control over the recipient logic without touching the system event's own definition.