I think Flow people need to solve this for their extension, too. (They currently hardcode three versions of that text.)

I am told the Flow side of things is handled as bug 60704. Once that is done the changes necessary in MM should be minimal.

bug 60704 isn't quite the same. The main point of that one is to make it so that the default messages in Flow are not WMF-specific (and to implement a hook so WMF wikis can override them in WikimediaMessages). MassMessage probably still needs to implement its own fix.

Apparently this is the "wikimedia-copyrightwarning" message from WikimediaMessages that displays there. Can someone confirm? The qqx hack does not work as the page reloads after clicking "Preview".

On Meta it was customized, so it was a bit weird for MassMessaging purposes. I've deleted the local customizations, thus resetting the message to the default one, which now reads "by saving changes".

Maybe we should just use an specific message for MassMessage?

@MarcoAurelio if it still matters, I believe that warning shown is set by:
$m['message']['help'] = EditPage::getCopyrightWarning( $this->getPageTitle( false ), 'parse' );
A quick solution might be to get the current warning and just replace "By publishing changes" with "By sending a mass message".

That quick solution is unlikely to work well with translations. The hook onEditPageCopyrightWarning could be used to modify the copyright warning to a MassMessage specific version.