Most users have the "Only things I watch or am involved with" setting chosen (for their own sanity), and it seems that when their username is assigned to a custom field, no email is sent.  

Is it possible to create a custom field (ex: "Tester"), which would trigger a notification to be sent to the person selected?

That's something I miss too. So +1 for this.

Dear Christopher, dear Gregor,
Please create a feature request for this.

Kind regards

Matthias

We experienced the same issue. I created a Bug for it:

https://community.openproject.org/projects/openproject/work_packages/40169/activity