Turns out, at least a few folks have invented a term for it, calling it the "Five Percent" approach.
Blogger Noreen's Five Percent (which I stumbled across four or five links down into a read of some favorite eco-blogs) uses this as her premise:
Imagine if every single person on the planet invested 5% of their
energy in working toward a better self and a better world. I decided to try it,
since it's something I can do not just once, but every single day. These are the
stories of the little efforts I'm making while living my ordinary life. I'd love
to meet you along the way!
She's been offline since August, but her blog has some great back-posts, and a host of neat "impact" links, which are worth a look-see.
Another way to look at the 5% is what I call the "stone soup" effect. If you don't know the story of Stone Soup, you can read it here. But in a nutshell, but doing things one little bit at a time, moving reluctant change slowly and stepwise, it is possible to create something larger and more magnificent than anyone thought possible.
Easy Green is a reference of some of the things one person can do to do one's own 5%.