If you're really concerned about people not contributing back their changes, I'd first consider whether it'd be due to their unwillingness or maybe a cumbersome process of doing that.
For instance, I can't find anything on the website that would explain how to submit a patch. There's a mailing list and it'd be great by 2002 standards. Hosting this on GitHub (or any other site with a forking workflow that's just as smooth) would probably solve your problem without having to resort to imposing legal restrictions. There still would be a few companies/people unwilling to help but only a considerable minority.