I wonder if it's possible to hack an electronic air freshener in this way.

I've never known an electronic air freshener which wouldn't be more useful as a punishment than a reward.

Right now, I'm using Tom's Planner, a Gantt chart app, for one of my class projects. It's working really well.

I've considered that sort of thing, but I'm not really very good at estimating how long things take. I had considered building a time-estimation game into my scheduler, but so far that hasn't worked out.

If your typing speed has a strong correlation to how quickly you're getting your work done

Unfortunately, work speed only loosely correlates with utility. Bad code written quickly can waste more time than it saves.