I actually found a work-around. In the 3rd party "checklist activity" module (Moodle), it actually has a email notification feature. So if I carefully design the course, and have the checklist activity as the last activity (then enable the email notification once it's checked, see screenshot below).. then it would be able to achieve the effect that I had in mind.

Just want to share with everyone who are interested in a work-around. Hope this helps!