Turns out that he wasn't that far off. The Simpsons had taken a swing at the environmentalists. In an episode from November, 2000, titled "Lisa the Tree Hugger", Lisa meets a guy from a radical environmental group called "Dirt First", and ends up joining to help them protect a historic redwood tree. On of the jokes of the EarthFirst! spin-off is that the leader of the group named Jesse Grass claims to be a "Level 5" Vegan, refusing to eat anything that casts a shadow.
The writer behind the episode, Matt Selman, is a Simpsons veteran with multiple awards, including a handful of Emmy's, to his name. He was even one of the writers personally courted by Matt Groening to help write the 2007 Simpsons movie. In his own personal blog, Selman uses some interesting techniques, including some made up super villains to make some environmental statements.
He's not quite "Dirt First" material, but he's got a green side to him.