Here we need to set the mirrorSelector on the repositorySession
and repositorySession is not handled by populateDefaults
otherwise maven.getMavenProject().getRemoteProjectRepositories(); won't have the correct repositories...

to avoid setting the mirror stuff on the repositorySession we would need to use org.apache.maven.DefaultMaven.newRepositorySession( MavenExecutionRequest request ) 
But I don't know how we can use this method inside forge...see my other comment on that subject