Thanks for the suggestions.

Row level security will not work for us as the datasets are internally shared without restrictions. The pre-selection of the manager is just for convenience, not for compliance.

Making copies could work, but honestly I don't want to create hundreds of copies and automate the whole process just to send this email. 

Yes, a custom app could work, but the hope was to reuse the existing dashboard, instead of reinventing the wheel.

I'll probably raise a feature request...