It would depend what they get out of it. It wouldn't have to be called a union. A certificate, code of conduct kind of thing would already be interesting.
Hmmm. What about the second aspect of the GP though? A union is the epitomization of welcome-all/accept-all. I'm imagining something that imposes restrictions on who can apply for assistance, both to ratelimit overall demand and also to make it incredibly hard for scammers to get anywhere.
On the surface (ie, legally speaking), it would absolutely be an exclusive environment. The idea (and probably secret sauce) would be figuring out how to delineate between scammers and legitimate devs.