Negotiate with the organizations behind the FOSS projects if necessary. It's hard to say given the lack of historical precedence, have there been any situations when business operations were stymied by FOSS issues? Besides something like the left-pad fiasco?
OK, sure, negotiate. But there's a lack of two things: Carrot and stick.
Unless the person working on the OSS project is a member of the union, the union isn't doing things for them. He pays no dues, they don't poll him or count his vote. So it's like when companies demand OSS contributors are part of their "supply chain" and therefore must fix bugs on the company's timetable: You can't unilaterally foist an obligation on someone and expect to be taken seriously.
So if a hypothetical union I'm not part of demands I stop working on my hobby, they can neither hurt me nor stop helping me if I don't comply, so I'm not going to.
And given the ability of a single hobbyist, or a small group, to do outsized things in software, that seems like a pretty big hole in the concept of a union having a monopoly on labor in a given field.
What would a hypothetical developers' union do if it called a strike and people kept working on their OSS projects?