I've been trying to find a clean solution where these source folders are added automatically, but unfortunately, that doesn't seem to be the way that maven works.
The best I could do is to create two simple mojos inside of the groovy-eclipse-compiler plugin that adds the extra source folders, but then this information needs to be explicitly added to the pom.
So, this is what it will look like (but haven't pushed this to any repository yet):
It works, but it is annoying and not much better than using the build-helper-maven-plugin. I'd like to not have to include this extra cruft in the pom and call the new mojos implicitly. @Alchemist, do you have any experience with this?