I am not sure if there are any scientific material concerning the 20-20-20 thing, just some blog posts and a webMD page:
http://www.webmd.com/eye-health/computer-vision-syndrome?pag... . Its just something to try, as taking eye breaks every now and then is probably good. But, thanks for the feedback!
For more general purpose break management tools, theres some pomodoro (not sure how scientific that is either) time/break managing tools online, but I might add some bars or user customizable bars if I get around to it. I usually try to stretch or take some deep breaths whenever my 20 minutes are up. For something like going outside, you could just add a weekday timer/reminder on your phone/watch at foo and bar times of day.