Wouldn't the union only be composed of the same colleagues who were resigned to 'hushed mutterings in corners' and laughed at the grandparent poster for 'being on some kind of moral high horse'?

A better approach might be to have a consumer advocacy organization/site that allows devs to anonymously leak and publicly shame the bad practices of their employers. Somewhat like the proliferate Business Software Alliance ads that used to say, "If your employer is using pirated software, report them in confidence at ~place. If we catch them, you'll get a reward".