I'm a recovered bulimic, a martial arts instructor and an occasional Eating Disorder educator (I speak through a group that does awareness raising, educating speaking engagements for clubs, schools, colleges, universities etc.), so this is an issue near and dear to my heart.
There are a few basic things we, as martial arts instructors, can do to combat the underlying things that can lead to eating disorders (ED's)
1) Don't use food as a reward. Overall this tends to lead to "medicating" with food and assciating it with achievement, rather than nutrition.
2) Don't tolerate size-ism. Don't let people make fun of the shape or size of other students.
3) Emphasize fitness rather than body size. If you get people to worry more about how they train and what they can do rather than what their bodies look like, it can have a positive effect. The thing to watch out for is when people start associating body size/shape with performance. While the two CAN be related, they aren't always.
4) Emphasize nutrition and caring for ones body in a compassionate way. There are a number of people who don't always make the connection that martial artist == athlete and we do need to treat ourselves accordingly (resisting overtraining, balanced food intake, getting proper rest) and as instructors/coaches we have a certain responsibility to impart these ideas on our students. If you don't have the expertise yourselves, see if you can bring in someone to talk about it as part of a class.
5) Remember ED's aren't just a female thing anymore, they're a human being thing (unfortunately) so don't just target one sex for positive body image reinforcement.
6) If it's something you worry about, educate yourself. As was stated earlier, there isn't a lot you can do, but there are a number of things that be helpful by not doing. My favorite resource is
http://www.somethingfishy.org/
If I can be of any help, please feel free to poke me, I'm always happy to assist.