@pastanaga - I had a friend who worked with a travel agency who organized those kinds of trips and she said they are becoming more and more popular. There are a lot of people in the world who want to contribute to efforts to preserve the environment, but aren't sure what to do outside their own sphere of green living.
She said one of the most popular trips was working with sea turtles in South America and I can see why. That would be an amazing experience even without being able to feel good at doing something that really means something.
That said, I would like to point out that another option for green travel is to go and see something new in your own country. Very few people have explored everything there is to see in their own backyards and it certainly saves on the air pollution if they do so.