I don't know of any unions established in the more recent Shadowrun timeline books, but something to bear in mind is that corporations are themselves nations. That means that a given corporation might have some sort of corrupt image of unions. You could consider some of the more "liberal" AA corporations as possibilities for this. Maybe Gaiatronics or Draco Foundation or one of the newsnets. But what I'd probably consider experimenting with is Ares Corporation. Why? Based in Detroit, and incorporating GM under the Ares umbrella, there could be some sort of corrupt, twisted, and distorted remnant of the original auto workers union working there. That could be interesting to play with. I don't know whether the union would be relegated to GM alone. Law enforcement is well known to have unions, so that union culture could then extend to Knight Errant. Would GM and Knight Errant then have the same union under Ares? Probably not. So you've got these two unions under Ares, each twisted in its own right, and probably carrying on petty squabbles against one another at the same time.
The effectiveness of these unions would have to be questioned. Most Ares employees have absolutely no options or mobility. A KE guard can't necessarily pack up and go to Lone Star. Or can they? There have been instances of security forces strikes in Shadowrun history, so that would indicate some union presence (although I don't think Lone Star or KE have ever gone on strike) and in the changeover of contracts from Lone Star to KE in Seattle, KE picked up many Lone Star officers. So there is some mobility, at least when Ares is involved as the acquirer.
There are definitely options to explore and imagine with it.